I've seen a variety of packages offered, but here are a few examples. Percentages vary according to geographic location with rural often offering higher as an enticement away from big city life.
1) 33% of associate collections including exams for hygiene checks, associate pays lab fees above (x) dollars pre-tax
2) 31% of associate collections including exams for hygiene checks, practice pays all lab fees
3) 40% of associate collections, does not include hygiene checks (but hey that's where you find your operative/pros/etc), practice pays all lab fees
One practice that I know set up an escrow agreement for the buy-in. They set it up like option 3 above, but only paid 35%. The remaining 5% was placed in an interest bearing escrow account to be used as downpayment towards the buy-in. If the associate decided they wanted out of the buy-in, they forfeited half the account (to cover associate position advertisement and search costs). If the owner wanted the associate out, the associate received the full amount minus taxes.
Given the difficulty some kids will be facing obtaining practice credit (due to tightened markets and the size of student debt), some practices are doing equity buy-ins. A portion of the associate compensation goes towards the buy-in cost until the agreed amount has been paid.
Hope thats helpful....