I live in between DC and Balt. In this area the standard Craigslist price for a gently used bikeshop BMX bike is $95 to $150 for entry level rides. I recently bought a Haro X2 that had never been ridden for less than $100. I don't think it was a rare deal.

I would advise you to refrain from sinking $ into a recent model Mongoose unless it is one of those super rare pro qualty bikes that make up like 3% of their sales. Mongoose sold out in the 90s and became a maker of discount store bikes. Sell your Mongoose at a yardsale. Don't wasre money on new parts. You'll never get your money out and the bike once fixed will be heavy and weak. Plus no one will think it's a cool bike.

If you're talking about a late 80s Mongoose then none of this applies and you should restore it with vintage parts and then go on Antiques Roadshow.

Good luck.