My cosponsor's letter showed an actual amount, in addition to a copy of her bank statement, which proved that she could actually afford to give me the amount she promised.
You might get away with a cosponsor saying they will give you whatever money you need, rather than an actual amount, but they are going to have to back it up with a bank statement. If you feel that you and your fiance have enough money on your own, and don't need a cosponsor, then don't use one. But if you do use a cosponsor, they are going to have to be able to back up their claim that they can help support you financially. The Consulate is going to want to see dollars/pounds, not just a vague statement that your cosponsor wants to help.
I don't understand why mentioning amounts would hurt. All that could happen is that the person applying would be told it's not enough money (which I don't think is going to happen in your case). But in a case where the cosponsor really wasn't offering enough money, and the applicant didn't mention the amount, the Consulate wouldn't just automatically approve the visa because they didn't know how much money the cosponsor was offering.