Take a look at what artists get through residencies in your state or even better ask your artist what he/she would accept in return for her valuable time. The Pennsylvania Arts Council has a listing of artists available and the "going rate" to have them in your classroom. This information is probably available on line. You may want to preface a limit you have to spend, possibly stating up front that what you have to offer is X amount, then ask what time frame the artist would be comfortable with.
Congrats on getting the grant. that's so exciting!!!!