We just had CT based authors Jessica Haight and Stephanie Robinson (who is a fellow LMS on here) come to our school for a wonderful author visit. They write mystery novels perfect for the grades 5-8 range. See link below. 

They spoke to our whole sixth grade for a block, and then they met with about 30 students for block four and held a writing workshop. They were entertaining and fun. I loved what they shared about working together and about the writing and EDITING process. (Kids never want to edit!)

I did not book them myself. so I cannot speak to their cost, but our sixth grade teacher felt they were reasonable.