We don't have a lot of disposable income here, but I've recently bought myself a 10 inch lenovo tablet so that when the kids are hogging the TV or just generally being a PITA, I can hide in the bedroom with a cuppa and watch a film by myself (we don't have a TV in the bedroom), or lay in the bath and watch Netflix etc. Neither DS may pinch in when their kindle fires have ran out of battery. It wasn't a fortune, its not an iPad, but it's money spent just on me, and that doesn't happen often!
If DH is away (as he often is) I've also discovered a lovely FB page who deliver pick n mix to your door 
What do you spend money on that is a treat just for you?