Is it negotiable between purchaser and buyers agent?
I think you mean negotiable between the Buyer and Seller.
A Good Faith, or earnest Money Deposit, is there to demonstrate that a Buyer is serious.
It's there to sweeten the deal, or make that Buyers' Offer look as attractive as possible.
It ought be large enough to appear as being too big for this Buyer to simply abandon in case he were to change his mind, or find something better. In other words, "Big enough to hurt".
I don't know what the usual and customary deposit is in your area; but if you were here, $5000 to $10,000 would be sufficient to show that you're serious; but more would look better. With a Conventional Loan, you're going to be putting $50,000+/- down anyway, so why not have the same money do double duty?
We used to have lots of people tie properties up with a measely $100 or $250 and then walk when they got back home or saw something better. Too little to chase them for.