This really is different for every individual.
Years ago, I saw a guy in line at Costco to return a pencil sharpener. The thing cost about $20. Forget the gas. I recognized the guy, he was a mutual fund manager with a million dollar plus salary. But I guess he valued his own time at far less on Sunday than the $500/hr he made during the week.
As your question suggests, the starting point is your wage, but for extra money, it goes down from there.
The question has merit, I just think that few people stop and do the math. There are also some things people just enjoy doing, like mowing a lawn, which I'd rather pay for. Often, people will pick and choose that way and not really decide whether they are working for themselves at a certain wage.