StayOnBudget is a pleasant, clear-use, common phrase, exactly what end users are looking for to brand. Not exciting but clear and serviceable. Dot.net and .co.uk also taken.
Might take awhile to find the right buyer but a good end user email campaign should do it. I'd email a few hundred at least.
End users are in the financial field, financial planners, personal finances, business planning, debt help, etc, and this is an attractive name for them, so someone may dig into their pockets for it.
Hard to value though, I'll give it a wide swing:
If you have a difficult time getting buyers, you should still be able to sell it for mid to high xxx; if you're good at finding end users and negotiating, anywhere in the x,xxx range low or high. High is of course very optimistic and assuming you found the perfect buyer.
Hard call to make, with a phrase like this. Nice one, could sell instantly, or could take years, because it has some pros and some real cons: pro is that it is the boiled-down, purest version of these kinds of phrases; con is that there are many, many variants, and if you ask too high a price many might just want another variant at cheaper, or reg fee. Other variants are like stay on a budget, stay on your budget, staying on budget, staying on a budget, stick to a budget, keep your budget, etc etc
But you've got the purest one, and companies like that for branding...
GL