I have three older diesel heaters, the drip type.
Don't know if you would be interested in that type of heater. They can be messy and bothersome to light, but they can be found for cheap.
I bought all three at farm auctions. I only remember what I payed for the last one. It cost me $35.00. It's a Coleman and puts out around 1,000 btu.
The other two were in that ball park price wise. I don't recall what brand names are on them. None of them are here so I can't even check.
They are up north with my trapping gear.
I used them in my line cabins.
If you aren't in a hurry and go to farm auctions, they are a possible cheap alternative.
I also see the small milk house size propane heaters sell at those auctions.
They ususally fetch a better price but $100.00 should get a good one.
That's another possible less expensive option.
Even a heater out of an old camper might work.