Hi,
You should make reservations for 5 nights since typically check-in time in most places is 12-3pm.
The places nearest to the Las Vegas Convention Center in your budget range are:
Circus Circus is across the street but geared toward families
Stratosphere Las Vegas is across the street but still on the strip.
Tuscany Suites is off the strip and provides free parking but is more than your budget. Tuscany is good since you can load several people in one suite style room.
The drawback its not so close to the SEMA show.
Are you renting a car or do you plan to take taxis or public transportation?
Are you looking for enterainment ideas? if so what do you like to do?