Every single product, situation, customer, etc is different. So take everything including mine with large grain of salt.
1. Tell / Ask them to write out all the details of the requirement. If there are any that is any different to your current plans. And seriously consider if you can even fill those needs.
2. Ask if how much are they willing to pay.
3. Tell them you have something double of Point 2 in mind because of X.
And if you are a small SME, you should read up on 37Signals / Basecamp how to deal with large customer.
Basecamp has a really simple pricing for companies. All companies pay the same price, no matter how many users.
It makes all companies equals, none can influence the features/business. If you don't do XYZ we leave with our 5000 users. At Basecamp they are just another client. No pressure.
1. Tell / Ask them to write out all the details of the requirement. If there are any that is any different to your current plans. And seriously consider if you can even fill those needs.
2. Ask if how much are they willing to pay.
3. Tell them you have something double of Point 2 in mind because of X.
And if you are a small SME, you should read up on 37Signals / Basecamp how to deal with large customer.