I just completed my Sport certificate in November. I'll suggest a slightly different way of looking at the expense and also a different perspective on your training.
Look at it this way - after you finish training, you're still going to fly, right? That won't be free. Presumably you'll be renting a plane, at least for a while. Even if you buy, you'll have hangar expenses, gas, maintenance, etc.
So,...
Decide what you're willing or able to spend per month for your flying as a long-term,
continuing expense. Then make that a base number for your monthly training expense and add the cost of the instructor to it. That's a reasonably small delta, all things considered.
That will mean you'll be training at roughly the same frequency as you will fly after training. Depending on your budget and plans, that might be a slower than "optimal" rate for learning, and therefore it might cost you a little more total. BUT, you'll naturally reach the point where your skills don't atrophy badly between lessons and therefore they won't atrophy badly between flights after you finish.
And when you get your ticket it will be easy to afford flying at the same frequency.
Personally, I think this will be a
safer rate for learning. If you spend at your pain threshold, fly as often as you can possibly afford, you'll get your certificate quicker. Then you'll lay off for a month or so letting your bank account recover, and when you fly again your skills will have fallen off. Your primary goal should be to
maximize safety, not minimize expense.
Just a different way to look at it. And worth what you paid for it. Or less.
