But taking a concept with which you're familiar and building it out for a new purpose should be a good short-term approach if you're able to pull it off. You've got $60k, which is a good sum which gives you a nice chunk of time making it easier to earn the next $60k, which makes it easier to earn the next $120k.
So you earned a good years salary, in under half a year, with knowledge to apply in new projects.
Is there another business opportunity in your client's industry, or another industry with which to apply similar toolsets you just created? You have the luxury of a bit of time to make a decision.
I'm inspecting now for other opportunities within the industry. This was fairly customized for a very specific need, so it still remains to be seen ... I also do not want to distract myself from my current startup, but I'll take the opportunity if it arises and I can manage it!