Caroline -
You have probably concluded that a fair number of folks are a bit biased about anything from Wally World (which is a bit of an extreme position, but it is what it is).

I do think you can get a good $150 bike at Walmart for about $100. For most folks it is the right choice. Don't sweat the choice or opinions of others.

Now if you have time to shop, then you could drill into local bike groups (ask at a bike shop for leads) or even online into craigslist and probably find a nice lightly used $300 bike for about the same $100 (search on Trek or Giant or similar bike names) or you maybe for only $50 if luck is shining on you that day. Other option is to visit a few bike shops and ask them if they have any good used, repaired or traded-in bikes they would be willing to sell... But don't let them know you have only $100 to spend. If they show you some available bikes and one of them looks right to you, then fit on it, but look disinterested. Then ask about their best price for it. Remember that any bike not at new and full retail condition is pretty much open to haggling on the price; nicely at first. If you can get a bike that works for you at your price point, tentatively say you would like to do that... but ask if one of the bike mechanics could give it a looking over to make sure it is ready to ride and safe. Bet there Mech will do it right for you. Being pleasant and friendly in your dealings can be helpful.

Hope that helps
/K