To many variables. What all gear necessary for a dive will cost, or gear for the lessons? Depends on purchasing location, and brand/type.
You can get all the gear necessary to go on a dive trip for under $1000, but will it be the same gear you will want a year later?
Rent it until you figure out what you want.
Our local shop charges $40 to rent everything.
For many open water classes you need only mask, fins, snorkel, and boots. When signing up low budget students, I have got them into decent gear for around $140.