I've had real good luck at the local dump. They have a clearance yard for used bikes. $5 each. They are usually missing a part or two and have some rust... But any BITOG'er should be able to fix them easy.
My BIL and I sourced one for his son's GF's kids. Took two $5 chassis to make a good one. A Trek aluminum frame with suspension for maybe $20 total
I just finished an Aria aluminum frame for the Grandbaby from there (well not baby, she's seven ...). Have maybe $30 in it with a tire. And I did a Trek 850 chrome molly frame 26" with an Italian leather saddle from same source for her mom when they visit next. That one I have over $100 in it as it needed all new cables and full set of front brakes. But it's very light and quick
That's nothing compared to what a similar weight bike would cost new ... The WalMart bikes are OK for just around town. But if you want to really go anywhere, weight and bearing friction will be your enemies. That Trek 850 has THE smoothest and free'est bearings I have ever turned. And it is all light weight. WalMart bikes look cool until yu get into the details ...
I have been bicycle riding since I rode with the Reliegh team out of Kezar Cyclery back in the '60's on a Super Course (fine bike), and a Grand Prix as a daily trainer. My current dirt/gravel bike is an aluminum framed Trek, and my slick road bike now is lovely older Peugeot
I still have my 1957 Schwinn frame/forks. Don't know if I'll sell it, or do a resto-mod on it ...
Point being you need to rent some from reputable bike shops and see what you like? Cruiser, Mtn Bike, 10-Speed, whatever. Try them. A good bike shop will adjust their rental so you are riding well and you will soon see what appeals to you. THEN start shopping Craigslist, thrift stores, the Sheriffs auction, etc. You will turn up a couple of keepers for less than one new bike including you rental fees...
Recycling is good thing. All the money you spend on repairs and upgrades stays mostly in the local economy. Your local bike mechanic will become a friend of yours. When you finally decide to do the BIG $$ all carbon fiber super duper, they'll help you find a trade-in for 1/2. That'll still be $5 large, but it won't be $10 ...