Tom makes a great point about ensuring that your Realtor and lender are communicating. I have been involved in a couple of deals this year where the Buyer has left money on the table because his Realtor overshot the mark in Seller contributions.
When I represent a Buyer who is going to ask for Seller contributions, I start talkng about this strategy with the lender early to make sure we are on the same page, and that whatever is requested by the Buyer can be fully applied to the loan.
For budget planning, Tom's numbers of 3% conventional/6% FHA have worked very well for my clients, but it is very important to pay attention to the details behind these percentages once the negotiation starts to make sure the full amount is realized.
Hope this discussion thread has been helpful.