Typical Finders fee value and conditions

What have you seen / been practicing in terms of finders fee?

I was thinking in terms of up to 0.5% of the transaction with a cap of $50k.

Naturally, a few conditions would have to be met, like: 

- only applies to successful transaxtions

- need to arrange a meeting with the seller within. A specific period of time

Anything else? Any comments?

Best