I agree but would like to go to a bit more detail and single out some artists. I'd like to share my commission experiences here and one that never happened.
I dealt directly with Joe Rubinstein. He accepted less than 50% upfront and delivered within two months. Joe sent a bonus which are the Green Arrow pages and the Spider-Man pinup. All of these covered two separate occasions of dealings all of which turned out great.
I also dealt with Mike DeCarlo and he completed the commission in two weeks. For this one, I had him ink over my pencils. He took 50% upfront but his rate was very reasonable and time was fast. I actually just contributed in another thread in this forum with my experience and process with Mr. DeCarlo. Check it out here:
But there are also times when I deal with agents. I dealt with Frank McLaughlin but he did not have Paypal and I coursed it through his friend who took the agent role, Russell Rainbolt. He is also the one who sells Mr. McLaughlin's work on eBay. I paid 50% upfront and the commission was done in two weeks, and it was huge art. Here it is:
Then there is John Byrne. I dealt through his agent, Jim Warden. Even with an agent, Mr. Byrne did not take any money upfront. He took zero money and would not accept payment until he finished the commission. Mr. Warden on the other hand did not get anything also because he knew me from previous transactions. It was done in less than two months and came out to be a beauty. I only paid when it was ready to ship.
I also recently did my first commission. A collector saw my works posted in my CAF and here, so he asked me to do him a recreation. It was my first time to get paid so I told him I will not accept a single cent until I am done with the art and I have shown him a scan to his satisfaction. The end of the story - the art is with him now, I am fully paid and we are both happy. I completed the art in less than 3 days. Here it is - Captain America 12 cover recreation.
Finally, there is one artist who I have been trying to get a commission from through his agent - whom I have transacted with a few times in the past for original art. It would have been one of my "birthday commissions" which is to get the artist to recreate or reimage a cover to an issue he was a part of cover dated on my birthdate or October 1977. Well, the agent asked for 100% full payment and quoted me a longer lead time than I am accustomed to, plus the price was quite steep - approaching John Byrne levels. I just could not get myself to close the deal.
So the moral of the story: don't pay 100% upfront and if that is what the artist or agent wants - then don't deal with them or run the risk of never getting your art.