Sponsors will give you a payout and a bonus at the 6 month and one year mark every year. This payout is equal to the sponsor amount plus a bonus that is proportionate to the ad effect you generated over the past 6 months. The early sponsors can easily be maxed out in 6 months resulting in the bonus being higher than the sponsor amount.
Once you max out an ad effect for a sponsor, you will get a gold star, reward and an introduction to a new sponsor. As soon as this occurs, drop the sponsor and add the new one. Sponsors only give out one reward and introduction so there's no reason to keep them on after you get them.
Ad effects carry over year to year, so if you don't max out a sponsor in one year, you'll likely get them the next year.
Here is a list of sponsors and their rewards:
|Bridgerock||80||80.0K||Knobby Trs plans|
|Mocha Cola||130||199.0K||Hyper Oil plans|
|Curry Coop||180||297.0K||40 Research Points|
|Froggerade||205||330.0K||Drag Race training method|
|Spongecorp||230||355.0K||32 Bit plans|
|Race Trax||255||375.0K||Emblem Plans|
|Cheapy P's||280||387.0K||Car Camera Plans|
|Dairy Duke||305||414.0K||Pwr Slide training method|
|Dull Comp||330||426.0K||Rocket Sci training method|
|Milky Milk||355||452.0K||Gold Egg plans|
|Bob's Drugs||380||465.0K||Watch vids training method|
|AD&D||405||515.0K||Aerobics training method|
|Chimp Labs||430||320.0K||Supercharger plans|
|Pear Inc.||455||400.0K||4WD plans|
|River Bank||480||550.0K||Shop Class training method|
|Honza Cars||505||400.0K||Carbon Brakes plans|
|Nights Inn||530||450.0K||Gear Shift training method|
|Autoburg||555||542.0K||Duck Car plans|
|Microsloth||580||675.0K||Drift training method|
|Kairo Group||630||1175.0K||SL Car plans|