Gas Costs from Brownsville,TX to San Jose, Costa Rica

Hello Robert and fellow travellers (sorry this blog is a long one, but well worth it for new travellers to read).
This is my first entry and before I talk/ask about gas costs, I just I thought I would applaud everybody for making the effort to tell everybody about their problems/advice/highlights of their memorable treks on this site and to thank Robert for setting up this site. It must be a lot of work answering all these questions, especially the same questions over and over. lol.... But there is a reason some questions are asked repeatedly - i.e. gas costs, shipping car costs, border issues, police encounters/safety etc - without the answers to these issues and feedback from fellow travellers and Robert, many would-be travellers might be discouraged from travelling altogether. So having accurate and reliable information from former travellers is key to planning a trip and not giving up on it.
Personally, I have travelled to over 80 countries. In fact I am the former author of 'EUROPE ON 84 CENTS A DAY' (my book is not published anymore, google the title) and I was a guest speaker for 22 years at over 300 U.S. universities, so I am a bit of an expert on budget travelling. In fact, when I was 19, I hitch-hiked in 1976 across Canada in JANUARY!!, down the U.S. coast where I attended the Patty Hearst trial, through Baja, over to Mazatlan, along the coast, Mexico City, then through C.America to Panama - that trip only cost me $300 for 3 months!! lol, plus the airfare from Panama City to Miami. I feel the more you can stretch out your dollar without sacrificing your health and enjoyment, then this allows you to see more countries and travel for a longer period. Most importantly, as a student, if you can meet and stay with the people and return their hospitality by leaving small gifts or during small chores, then you will learn more about the culture than you could on any organized tour.
I have still travelled over the years, and especially the past 4 years with my son to tennis tournaments all around the world including China and Russia. He hopes to turn pro in a couple years, but first he will attend a U.S. university on a tennis scholarship this August. My plan is to drop him off at the school and then continue to Brownsville and follow this exiting itinerary on my own by car:

Brownsville,TX
Mexico - Matamaros, San Luis Potosi, -Guadalajara/where my brother lives, Puerto Vallarta/Ixtapa, Manzanillo, Acapulco, Mexico City ( i will leave car outside of Mexico City - people have been fined by the crooked police in Mex.City based on not being allowed to drive during certain hours and their license plates or I might skip Mexico City altogether) , Puebla/Cholula, Oaxaca/Monte Alba/Valley of Mitla, San Cristóbal de las Casas/Palenque - (Misol-Ha and Agua Azul waterfalls); Merida/beach town of Progreso and see the Dzibilchaltun/ Uxmal/Cuzama/Izamal, Chichén Itzá , Cancun, Playa del Carmen, island of Cozumel
Belize- Caye Caulker,,San Ignacio/Mountain Pine Ridge
Guatemala - Flores, through Poptún and stop at Río Dulce, a small town on Lake Izabal, Coban, Antigua/ Chichicastenango / Panajachel / Lake Atitlán, Guatemala City,
El Salvador - beach area near La Libertad, San Salvador,
Honduras -Santa Rosa de Copan, Roatan Island, The Bay Islands, Tegucigalpa
Nicaragua - Granada, Ometepe, an island on Lake Nicaragua
Costa Rica - Monteverde, La Fortuna / Arenal, San Jose,Puerto Viejo de Talamanca ( then sell car in San Jose)
Panama - (wont' try to sell car in Panama- not worth due to huge duties), take bus from San Jose to Panama; visit Bocas del Toro (consists of 6 islands) Boquete, Santa Catalina, Panama Canal/Panama City - fly home

As you can see, this trip will require a fair amount of driving, so here are my questions. I have read some blogs where people (and one of them was Robert I believe) said it cost about $3.000 U.S. to drive from Browsnville to Costa Rica. I realize gas costs more, but that seems like an astronomical amount of money...I hope the cost would be a lot less for compact cars. I believe your vehicle was only getting 15 miles to the gallon??!!!
1) but based on driving a compact car, can anybody tell me, approximately, what their gas costs (diesel vs gas) were from Brownsville through Mexico (ignore my complex Mexico itinerary), Belize, Guat. El Salv. Honduras, Nicaragua, Costa Rica????

2) I have read in these blogs that Toyota and Honda cars are the most reliable, pretty gas efficient, and the most common car in C.America (which makes it a lot easier and cheaper to get parts/repairs). So I am trying to find a Toyota or Honda car that uses diesel. I have been calling dealerships in the Niagara Falls, Ontario area where I live, specifically asking about diesel cars, but they say they haven't been made for over 20 years!! I could try and get a Volkswagon diesel, but as the blogs have said, I would have much better luck selling a Honda or Toyota. I plan to sell the car in Costa Rica (I won't take it to Panama). My questions are: will I notice a big difference in gas costs with regular gas vesus diesel/ If I buy a car that uses gas, will i have MORE difficulty selling it in Costa Rica versus a diesel car?

3) One blogger wrote: "Thanks Robert for the reply. We managed to sell the car just south of Dominical in Costa Rica. It cost us $3,200 in California. It's book value in Costa Rica was $6,000! We sold it for $2,500 as the import duty on such an old car (1993) was going to be $3,500!"... so it really only cost him $700 for the car plus gas/tolls/bribes etc etc for this trip - not a bad deal at all) So, if I pay $1,500 in Canada for, let's say, a 1998 Honda, will I get around the same amount in Costa Rica from a buyer, maybe more?
4) what would the tolls cost from Matamoros/Browsnville to Guatemala if one was to just use toll roads?
If gas costs are too high, then I may have no choice but to takes buses. This is not what I want to do. I want the flexibility of stopping where and when I want. I may try to off-set gas costs by asking anybody at hostels/hotels who might want to share the gas/toll costs.

So if anybody, can give me fairly accurate gas costs and answers to the other questions, it will help me out tremendously with my decision: Car vs Bus.
Gil