Approximately 3,000 bumper stickers surface area equals the area covered by a large billboard. Billboard rental rates are based upon impressions, i.e. how many pairs of eyeballs see the billboard per week/day/month.
A single billboard can cost a few thousand to several thousand a month. 3,000 bumper stickers provided by me will run about $375.00 (including shipping). They will remain very presentable for the 16 months run-up to the election. Properly broadcast, 3,000 bumper stickers over the next six months will yield a much higher impression rate than any billboard campaign for a month. $375.00 divided by 16 months is less than $25.00 per month for millions of impressions.
A bumper sticker here or there isn't going to do diddley-squat to change perceptions realistically. To change perceptions there must be massive quantities of bumper stickers appearing on the street.
Here's a suggestion: get a batch of bumper stickers and go find some Ron Paul sympathetic fleet owners/managers. Place TWO Ron Paul bumper stickers (for two impressions in one spot instead of one) on each car/truck/van in the fleet. Taxi cabs, courier services, delivery trucks, service trucks, etc. all spend a lot of time on the road which equals more impressions than you driving back and forth to work every day and your running of errands.