If sponsorship were only so simple...
You are worth precisely what someone will pay you, not a penny more.

As for assessing value to your team in general, you have to ask some serious questions.
What championship are you running in?
Are you running a whole championship or just a few rallies?
If just a few rallies, are they all in a certain region of the country?
Are you capable of running in the top few of your class? Top several overall? Is your equipment? Does your equipment look good and professional to the outside observer?
Is the championship you are running in covered in the media? Where? TV? Mags? Which mags/channels?
What do you have to offer? VIP credentials at X number of rallies? A guide to take your VIPs to press stages/rally stages while you are racing? (Do you even want your sponsors around when you are racing? Pressure?) Title sponsorship of rally team? Logos on apparel, paint schemes on cars/service vehicles? Website? How many hits? Can you put together a corporate entertainment package for them that will knock their socks off?
Are you and/or the car available for product introductions, marketing campaigns, special events?
Can you keep them updated with fresh news after every rally or if you're not rallying, often enough to keep their interest and see value?
These questions are just scratching the surface, but I know of NO sponsor who pays money for a sticker right THERE and that is the end of it til next year. Sponsorship is a two way street that is a LOT OF WORK!
The answers to these questions will help you figure out who you should be targeting and what you really have to offer. If you can find where they are currently advertising and what they pay for what kinds of return they expect, you may very well be able to assess a value that makes sense to them. Always remember that you want to over-deliver. Tell them what they will get and give them more. They will come back if they see the effort is doing well for them.
As for the pitch, everything needs to be right. You need the right person, the right timing (preferably when they just found an extra 10 grand in their marketing budget

),the right company, the right presentation, etc. If you suspect you don't have one of the variables ready, wait until you do (timing is, of course, the hardest mark to hit).
Good luck.
edit...ps...Rally team owner/driver/
marketingVP/vehicleprojectmanager is the toughest job I've ever had, but its the most rewarding.