A basic conversion will run you about $150-$175 for parts and a few tools. You'll end up with a rifle that looks much like this (my rifle prior to the forearm conversion):
FYI - there is nothing wrong the the stock Saiga handguard. It's actually quite nice in that it covers the hot gas block and allows for a more forward grip. Replacing that handguard adds up. You'll need an AK-47 gas tube ($40), a new AK handguard set ($50 for a good US-made KVAR set), and a front handguard retainer ($50 for a bolt-on retainer).
You could save a bit of money with a foreign handguard set, but I wanted enough 922(r) compliance parts to eventually allow a foreign muzzle device. You could also save money with a pressed-on handguard retainer, but that requires removal of the front sight base, gas block, and access to a press.
Another option is a quad rail like the excellent (but heavy) Chaos rail for Saigas. It's an easy bolt-on part that allows you to use the original Saiga gas tube, and does not need a handguard retainer. It runs about $115.