Expect extremely subjective responses. The basic answer is "it depends".
How much would you be paying to get into snow skiing, including lessons, clothing, all new equipment (including goggles, helmet, camelbak and 2-way radio), and ski passes?
You'll likely drop between $1000-$2000 for equipment, depending entirely on whether you buy new, used, wetsuit, drysuit (which can by itself run between $1-2K), two tanks - steel v. aluminum, computer (not necessary but expensive if you opt to get one), etc. etc. If you prefer new gear, estimate to the high end. If you have access to used gear, you might still estimate to the high end... the savings are seldom spectacular...
If you're diving in local lakes, dive day expenses will largely be food, gas, boat costs if you have access to a boat, fills, and the endless miscellany that you'll need in terms of parts, service, annual vips, batteries, gloves that you forgot, etc. WAG = $50-$100@? If you're heading to a destination in Mexico or points east it would of course be much more expensive.
If you want to maintain and expand on fundamental skills, diving one or two days each month would be a good minimum, however this is going to be darned subjective also.
Scuba isn't cheap, but I've found its relatively comparable to sports such as snow skiing. Once you have all your equipment, annual costs level out to what it involves to maintain the equipment and actually go diving/skiing. In both cases, however, there is a relatively high initial acquisition cost to put together your gear.
FWIW. YMMV.
And by the way, welcome to the board!