Undeveloped: reg fee
Developed: depends on traffic
So right now, your best bet would be to develop it, then come back and ask an appraisal. It has great potiental (which doesn't mean anything until it's applied) and you could turn it into a flirting/date site via mobile to mobile. It's a cute idea, but can you do it?
However, if you hold on to the domain name for a few years when .mobi becomes more popular, the value will increase.
Just my 2 cents...