I ran into this while freelance designing for Playing Mantis and their Johnny Lightning line of diecast. I told them up front that I wouldn't sign any work for hire agreement. I was content getting paid my hourly rate, or per job, but did not want to sign all rights over. SHould they consider a re-run of any of my diecast designs, the term would be negotiated at that time.
'No problem, we'll work out the details later' they said. lol... Every year, like clockwork, their attorney would contact me about getting an agreement signed... Every year, the contract would have 'work for hire' or 'in perpetuity' etc etc etc. I'd tell them 'I'm not going to sign that'. Theyd say, ok hold on doing any more designs for us until we get this hashed out... a month would go by and everyone would be pounding on my door to get designs done for their deadline, and ultimately the powers that be would say 'go ahead and do em'. I wouldn't hear of it again until the next year... 5 or 6 years of that, and then they sold the company to RC2. They tried desperately to get the rights transferred prior to the sale, but didn't want to offer any compensation. So I still own the rights to all of those designs.
Whats important to know, is when you discuss % of sales, you are dealing with a wholesaler... which means you are likely getting % on their wholesale sale price, and not the retail sales price at walmart or whatever... Keep that in mind if you negotiate a %, and read the fine print...
Personally I prefer a per run fee. And JL's were 4$ retail vs Mattell's $1 retail?