I believe this hotel is at exit 25 off 1-77. The race shop exit is 36, so its about 10 miles away.
You're not going to have much luck finding a non-expensive hotel during race week. Have you checked around more than Huntersville?
As for a route to the track - usually the hotels provide directions if you ask at the front desk. I did a map search for here and it says to follow 21 to NC-73 to Poplar Tent Church Rd (becomes Poplar Tent) and goes out to I-85. I'd say as far away from I-85 as possible. If I were driving this route, I'd follow those directions to Poplar Tent and then turn off on Derita Road (which will take you by Ganassi and Roush/Fenway Racing shops) down to Speedway Blvd. so you wouldn't have the back up on I-85. Speedway Blvd is going to back up, too - but there are stop lights thru there to help the flow of traffic.