The least expensive compensated bridge available is from Grover. They're only about $5.00 US. Problem is that they're not too easy to find. Elderly Instruments carries tons of Grover bridges but not the compensated one. But you may luck into one at a local music store.

If the intonation is still off after you've adjusted the bridge, replaced the bridge and done everything else you can possibly think of, just hang a sign on it that says, "Hell no, it's not in tune! It's a banjo!"