How much are the tolls with the motorhome from Boston to Rapid City S.Dakota through I-90?

Answer 1

Better to take I-95 to New York and take I-80 across Pennsylvania. Save yourself a fortune in tolls across NYS.
Actually here is a toll-free option:
I-80 across Pennsylvania
At Youngstown, take I-76 to I-71
I-71 to Mansfield Ohio
At Manfield, west on US 30 to Merrillville IN
North on I-65 to I-80 (about 10 miles)
West on I-80 to I-29
North on I-29 to I-90
I-90 across South Dakota
Distance: Only 75 miles longer than I-90 (of a 2000 mile trip)

Tolls for New York State:
http://www.nysthruway.gov/tolls/toll-files.html

No tolls from Buffalo to Cleveland

Ohio Turnpike mileage calculator:
http://www.ohioturnpike.org/calculator.html

Indiana Tollroads/Department of Transportation:
http://www.in.gov/dot/motoristinfo/

Illinois Tollway System:
http://www.illinoistollway.com/portal/page?_pageid=57,1302473,57_1302480:57_1302491&_dad=portal&_schema=PORTAL

Wisconsin, Minnesota and SD do not have toll roads.

Answer 2

Not sure how much, but its not that much. Sounds like you might heading to Sturgis in the next couple weeks??
I'm leaving 8/5. Good luck