I'd figure a few different angles to get to a reasonable value:
top end price: What a professional would charge. Obviously, you should not request more than someone who does this all the time
For what it's worth, I generally offer my tenants to do work for me for cost of materials plus $10/hr ($20 minimum). Works out great for me if I put together a list of a few things and have it all taken care of once or twice a year for non-skilled things.
If it were me, I'd probably do it all the same way. Cost of materials that you would need, plus $10/hr (my guess would be around half of what a professional would charge for time only).
Maybe offer a discount if she's hot and single.. :)