Vern, just my 2 cents, but the best arrangement is one that both parties feel is a fair deal! There is no "standard" co-drive arrangement as there are a whole bunch of different ways folks figure out to make things work for their particular situations. Every co-drive situation I've had has been a bit different.
When I co-drove Tim Aro's 10AE back in 2000 we calculated out tire costs on a per run basis (e.g., a set of tires costs x dollars + tire mounting/flipping and we get y runs and thus arrived at the cost). We also split gas, hotel, then I bought a used tire trailer for him w/o him asking for it, and then I helped pay for the various parts we needed to buy when the car was malfuntioning on a weekly basis...though, he covered most of that cost.
When Tim drove my Spyder the next year we did the same thing in reverse, expect the cost went up since we changed to Hoosiers. We also split the cost of having shocks custom built for the car.
I can't remember what I charged Buckely back in 2002...$20/run sounds about right. I did the same for Donnie, but charged him $1,000 per run. After a full season he finally figured he was getting screwed, so to get even with me he foreced me to sell my car, bought that POS 2004 Spyder and made me drive it

When I started driving Donnie's cars we worked out a different deal. In addition to our arrangement, Chris & I have "pitched in" to helped pay for clutch replacments, misc. parts, and other stuff. For one or two event deals where I'd really like to have a co-driver in the car I usually cut folks a good deal (i.e., meaning I'm losing money). So, once again, there is not one (1) right way to do it and I'm aware of a lot of other arrangements.
Some folks charge as much as $500 per national event to run a car. BTW, that's the highest amount I've heard of.
A note of caution to car owners...it is very easy to underestimate the cost of letting someone drive your car, so spend a fair amount of time in calculating things beyond tires & mounting & flipping. If a car is really well set-up it can take a lot of money and even more time to get it right. Plus, having a second or third driver adds to your workload in that you're getting tires changed/flipped more often...and stuff may tend to break more frequently. Someone who wants to co-drive a really well set up car should be willing to contribute more than just the costs of rubber.
I hope that helps.
Eric