I bought a alpine m-500 amp new in box for 90 of craigs list and them my sub was 125.00 10" solobaric kicker running at 2 ohms and made my down firing box for 35.00 and free wiring from a friend so I was close to that and it sounds Great!!!
Some of it depends on your size, I guess. I just got an amp and box on Craiglist for $40. And I got a brand new Sony Explode 10" at Electronic Express for $30. I always go cheap. I really need to look into marine certified. I have issues when it comes to going topless...I keep forgetting to cover up the sub.
I gave up on subs after I blew my kicker l7 12 inch due to only the holy lord knows what. Mud rain or my addiction to music. Those suckers aren't cheap. Kinda miss it though some days.
That's why I got a cheap setup. I'm 130$ into it and I have a spare sub. One just stopped working so they only sell 2 replacements for 30$. If I do happen to damage my subs with water, I can just set up the one good one I have and get more cheap replacements.
Yesterday I installed two 5 1/4 Sony speakers in sound bar. Two 4x6 in front. Then got a steal at my local pawnshop I got two mtx 8s in a ported box custom made for a wrangler trunk. Then a true 400watt amp made by Kenwood and a dual deck all for 100bucks and the other speakers another 100 though and wiring and mounts for deck for 40