A new 300# BB set at Play It Again is like $99, which means that Craigslist has them even cheaper.
Hmm, I think this unlikely. Sport Chalet and other sports stores generally carry these for $200. Maybe on CL, you'll find them cheaper, but I rarely see them under 3 digits. This also depends on where you are located in the US. Looking for a set for a guy I know in Oklahoma was very trying because of population. He would've had to drive 2-4 hours to find a set under $200.
At the local used fitness store, they do carry standard 1" plates for 30c/lb. Of course, I live in a city with 1-2 million people.
You may be able to pour two buckets of concrete or multiple buckets of concrete and then stand on something to get better depth for something like a trap bar DL.
Bag of sand at home depot is 5 bucks. Get some ducktape and you have yourself a fine sandbag.
If you made the molds, I'm sure you could pour concrete and stick some sort of handles in it to do DL with.