As for the companies publishing/selling your kits, they do all the hard work involved in producing/selling/marketing, so you can sit back and get something back if your idea is good enough. If you want to make a bigger margin, then by all means do it yourself, however unless you have a high margin product and popular item, then chances are your overall return is not likely to be that much once you take into account all the costs.
This has been talked about before, but the rule of thumb for margin in the one-man-band kit/product business is typically between 2.5-3 times the product cost. Absolute minimum of 2 times. So if your widget retails for $100, you'd want to be getting it made for $40 tops. And that would include a reasonable cost for your labour as well.
But that's only if you want to sell it yourself and keep all the margin for yourself. If you want to sell it through say Adafruit or Sparkfun or someone else, they are going to want to make say 40% on your widget, that means you'd sell it them for say $60. (rough figures).
Your profit has now dropped from $60/unit to $20/unit, and you are still doing all the work getting it made kitted up etc.
So at that point you'd be better off looking at getting them to do all the work making it and simply taking a 20% royalty of retail price with no work on your part (maybe support, because it's your design)
Or if you still want to do it yourself for satisfaction or control reasons, then you'd want to make sure that extra sales generated by selling through a bigger higher profile company is worth it. Otherwise you are better off selling it yourself.
So it might come down to say making $20/unit for no work, or $60/unit for all the work on $100 product. Your choice.
Jameco are offering half of that figure it seems.
The $ get quite small when you are talking say sub $50 retail products or kits. To the point of it being almost pointless to get a royalty on say a small $20 kit. Let's say you got 10%, or $2/kit royalty, you have to sell in the 10's of thousands to make a decent amount of money from it. Even at 20% royalty, that's only $4/unit which isn't much. FYI, a reasonably successful kit will sell in the several thousand, with only a few achieving 5 figure sales.
But at those levels for say a $20 kit, it's really not worth your own time doing it yourself, you'd be working for slave labour rates, even at a very high 60-80% profit margin. Remember all the time and effort it takes to test, pack and ship etc, especially overseas.
Dave.