It's always the same question, do I get new replacement windows or not. Am I being duped or not is a question you need to figure out by researching your options.
Replacement windows are a good option when all others have been done. Insulating your attic, sealing off attic hatchways and pull down staircases for winter, and weatherstripping always give you more than installing replacement windows. Unless they are falling out and have damage, I would paint and install some weatherstripping and caulk.Last resort would be storms windows as Glenn suggests.
My suggestion is to leave it until you have enough money to change the entire window assembly and upgrade the insulation surrounding the window unit. You can also seal the exterior and fix any head flashing issues. This does not always get done with replacement windows correctly. When I do thermal imaging, it shows up right away.
I'm not a big fan of replacement windows unless...they are done by a competent installer and those are hard to find.
It all really comes down to finding a great installer, every time for everything.
Sounds like you are one of those folks, I can say that because you are here taking your time to explain things.