I think a budget would help, but I don't think you're going to get out as cheap as you hope without sacrificing some sort of quality.
I had an in-ground post with a fiberglass backboard was at the end of its life. There wasn't an existing backboard that was really compatible with the post's elbow & bracket, so I ended up pulling it out and replacing with a taller section of well casing that I had on hand. Then I mounted this goal (
Link). I sort of wish I had considered a complete post/backboard package that would allow me to change the height of the rim, but the route I went ended up being much cheaper for a mid-quality level backboard. The well casing also gives me some flexibility in the future when this backboard needs replacement.