Congratulations on having the company recognize that you have a wonderful image. As you have written, the quandary is what to charge. A few ideas do come to mind....
First a story. Back in the 1800's one of the founders of Seattle, Washington was up in Alaska, saw a Totem pole, and one night made off with it. It wound up in Pioneer Square in downtown Seattle. In the 1980's a drunk burned it down, so the city fathers went back up to Alaska, and commissioned a replacement for something like $5,000. When it was done, the tribe notified the city that it was ready. The city came up and paid the tribe, who promptly told them that they were $5,000 short. The city officials dumbfounded said that was the agreed upon price. The tribe said - yes, but you just paid for the first one - stolen from us a 100 years earlier, you still need to pay for the replacement - the one we just carved.
Some items to consider:
- Regardless of how Cheeseburger came across the image, they see value it it - which is good.
- What is their business model - apparently they sell prints. They must have an estimate of how many per year that they would sell and for what price. You could ask for a commission per print, and hope that they will be fair in documenting the actual number they sell.
- Are they asking for an exclusive - if so then you could not sell the print on your own website, hence you would need to charge them much more - since they would essentially wind up owning the image.
- I would suspect that they would go for a non-exclusive one time fee, and that may tend to limit the sales you see on your website.
Here are some websites that have some royalty calculators...
... the problem is that it appears that none of them really address your specific need. It looks to be 120 units of your image from their website (over what time frame is uncertain).
I would ask them for some additional information, as in...
- Ask for an explanation of their business model - as in how they make money?
- How many prints do they sell per year?
- What is their average price?
- How many prints for their most popular image per year?
I would compare their information against what you are selling your prints for (and your costs).
Ask them if they would consider a per item royalty?
It might also be interesting to talk with some lawyers - in terms of what a copyright infringement settlement may bring you. They may be thinking that licensing the image from you would be a much cheaper way to essentially settle (since the agreement would probably include you forgiving them on the infringement issue).
I would think that they are essentially in the business of retail selling items that can carry a picture (clothes, stuff - coffee mugs, etc), that will move their merchandise. Maybe a $1 per item may be reasonable - with $2,120 payment upfront ($2,000 hand caught in the cookie jar fee included).... You have some experience off of your website selling the image. You probably have an idea of the margin you clear after costs - charge a percentage of your profit margin per image.
They may indicate that a one time fee would be preferable - then its a question of how high is too high, vs how low is too low.
The problem for you is that you don't know what is reasonable and customary within their business model and with the type of image you have.