Placement is a big factor for bass. Go to the cult and then poke around in the room modes section. There is an excel spreadsheet room mode calculator that allows you to get the best response at your listening position. Don't ignore the possibility of using two manifolds with two drivers each.
Pricing depends on many factors. I can tell you a low cost route.
Ficar Audio IB 18 or Mach 5 audio IXL 18
Now you have to trust me
Nady XA-900 amplifier (be patient on eBay you will find it)
Rat Shack four fan pack (you will use two in the amplifier for quiet operation if it is in the listening room. Not required if in the backspace.
Rane PE-17 equalizer
Rat Shack meter
For an all out system a Rane Balance Buddy or (equivalent from Parts express)
Any old flat stock for the manifold(s) 2X4's for edges and bracing if you want.
Nuts and bolts to fasten the drivers in the box.
XLR to rca plugs or XLR cables if you use the conversion box.
Heavy wire for hooking up the drivers.
Louvered metal vents from Home Depot to cover the opening(s)
Wire the drivers to present two two ohm loads to the amplifier. I defy you to stay in the room if you drive either of those 18 sets at even half power with the Nady. It is bulletproof and underrated.
Drivers will be the expensive parts.
Nady should be under $100
PE17 also under $100 (you can use the Behringer eq but beware of hum etc and it is a pure unadulterated bear to adjust as far as I am concerned)
Wood, panels, louvers, hardware etc. should be welll under $100.
What you will end up with for about $1300 or so, is something that will scare you if you value the structural integrity of your home.
If you are unsure, and presently have any sort of subwoofer at all, take the driver out of the box, cut a hole and mount it where you think that the more sophisticated system would go. Drive it with whatever amp was originally driving it. Don't worry about eq etc. Try it. I bet you pop for the full up system.
My very first IB attempt was a simple extract and mount of two el cheapo audiosource 15 inch woofers in a baffle venting into some attic space in the rear of my Home Theater. I just used the amps that were in the boxes. No eq, nothing fancy. I was absolutely blown away.
My first equalized IB with the Nady and a Rane used a couple el cheapo 15 inch drivers from Cerwin Vega that a chap on the cult sold to me for a very reasonable price. I just plugged them into the holes I had cut for that experimental IB, and was well under $500. It now resides in the bedroom theater. That original experimental set of el cheapo components is now resident in a manifold in the attic of my son's home. He is very very happy with that setup.
Once you unbox a driver, and give it room to breathe in an IB you will never again waste time and money attempting to build boxes that just stifle the cone in one means or another.