Ask for what is reasonable and customary first and be sincere. Your company should have some basic guidlines for what they offer for such. Then ask for more than that. Don't be afraid to ask for the rediculous, but be sincere about it. You don't have to see these people again (hopefully), you might just get it, and if your sincere about it they won't hold it against you later, should your paths cross or you have to use them for a reference. After all, chances are, they put you in the position for needing it to start with.