For a non-exclusive license, there's one easier number, and another one that's harder.
First the easy one: How many days/hours/months would it require a good programming team to rewrite the equivalent of your app? That number times a tempting hourly rate, times a development risk factor, would be their up front development cost without you. A lot of small companies without patents found out that their acquisition deal fell through when their stated cost of purchase was larger the the cost of big-corp ripping off the idea and doing it themselves.
Next: How much is value of time-to-market that they would lose by taking the extra time to develop what you already have available? I some markets, being first is worth 10X, 100X more than being late. Will a competitor beat them to market if they don't buy your code? You might be able to negotiate a really high price in that situation. If they would lose 3 months sales by not going with your technology, you could at least charge them a good fraction of those sales.
If you think there's a really strong upside to their branding, you may want to try and negotiate a percentage deal (with a nice advance royalty up front, of course).
Yet another number to consider is whether their branded version will eat into the market for your version. You've got to add a good estimate of that to your price just to try and break even. If you don't want/need the money, you could offer it at just that price, and end up with some extra resume fodder.
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