I don't sell Treks here, so I don't know too much about pricing. but just do some research online, and you will know if it's a good deal or not. Keep in mind, that even though $500 is a lot of money, anything less than that you won't really get a good quality bike. The components will be really cheap, no name, or made out of plastic. Try to look for a bike that is made in Taiwan as opposed to China. The price will be higher but you will be able to tell the difference in quality.

Think about it this way. Spend a little more on a good bike that fits you well and is fun to ride and that $500 or more will be put to good use. Or spend $200 on a POS and have it sit there in your garage bc you hate it, and that was $200 totally wasted.

Oh and let me warn you now.... you will have to break in those lovely sit muscles so for the first week no mater how bad your bum hurts just keep riding :p and here is one upgrade you might be interested in... A nice little shock for your bum if you are riding wooden paths.
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PS... some of the bikes you are looking at might have reviews here for you to get an idea... Mountain Bike Product Reviews - Mtbr.com

LOL I always forget to warn people about this! And this is what makes most people stop riding! I have a 40+ year old Brooks on mine so I kinda forget about that.

HAHAHA!!! I did a spinning class only 1x. I walked out with the sorest 'sit muscles' EVER and said NO WAY to ever doing it again!!! LOL That was the only thing that was sore for days on end, walking like cowboy so one cheek did not touch the other! LMAO!!!!

Anyways, our buddy has 2 Trek bikes he bought 20 years ago. They are garaged and off the ground, always have been. His (now ex) wife rode hers 1x and he rode his a handful of times, but this is 20 years ago. We are going to check them out and maybe borrow them for awhile to see how we like it this summer. If we love it and do it often, we'll go buy our own bikes (unless he just gives us his) I don't know much at all about bikes or bike technology, so no idea how a 20 year old bike would differ from new bikes, but I'm thinking if it was a good bike back then, it has 2 wheels, handles bars and pedals, it's worth trying it out right?

Yes! If he's willing to let you borrow them why not? Technology has changed but not much in the lower end of the market.

How long have the bikes been sitting? You should take them to your LBS and have then check them out. Most likely will need new tubes and tires if they've been sitting for a while, and maybe a light tune up.

I was thinking the same thing, then my brother (whos an avid rider with $6,000 bikes) said that new tubes and tires for the bikes could cost $100 each? Is that true? Thats not something I want to buy for someone else's bikes... I'd rather pay $500 and get 2 new bikes with all new tires! LOL

Maybe for his $6000 bikes, yes. But you have to think about what shop you go to. Labor rates can vary dramatically. We only charge $7 to put in a tire or tube. The shop down the street charges $20. Then tires can range from $15 for cheapy no names, up to $100+ for high performance; and the tube itself around $7 each. Then times that by 4 (2 tires on each bike, and 2 bikes).

Simple tune ups are also going to run anywhere from $25-70 depending on what you want done and if you need anything replaced (cables, chains, etc).

If you just want to try out the bikes for a day, see if the tubes will hold any air. It really depends on how long the bikes have been sitting. If it's been more than a few months I would really suggest getting at the very least new tubes. If your brother can help you change them out, then you don't have to pay the labor for the shop to do it for you. But since tubes are rubber, they can dry out. When you inflate a dried out tube, it will create fissures and air will come out.

However, the money you have to pay to get these bikes up and running will be about $100 or more, but remember that $500 will get you ONE new bike with new tires, so if you get two that's $1000.

Maybe you can strike a deal with your friend. Once you give him back his bikes (all newly tuned up with new tires) maybe he can pay you some of it back, maybe 50% since you got them fixed up? Or is he willing to just give you these bikes? If these bikes are in overall good condition (no rust or anything like that) then maybe it might be worth it to keep these old bikes. Check to see if the wheel spin true also (no wobbles). Trueing a rim can cost $20+ each.