Allow users to enter their own price

I am selling memberships which is fine with eCommerce when they are fixed costs. BUT what about if I want users to enter his/her own amount for example: a platinum member could be $1000-$2500 ; a silver $500-1000…is there any way to do this with commerce, Is there a better way? This will be a problem if I can only do pre-set amounts especially since some donors may want to give say $50000 or even $2,500,500 or whatever they feel like …What then?