Custom Build 21" subs

Have any of you made your own subs before? I was looking at prices of individual parts and I'm thinking I could make some sick 21" subs for about $500 each. I was wanting to upgrade to FU2000 SPL 18s but after taking the FU750s apart I realized how cheaply made they are. I can get a Quad stack 4" motor for like $250, the TI style 21" baskets with a custom welded 12" spider landing with triple medium linear spiders with dual sowen leads on a 4" 8layer team coil with tall roll surround. I am sure I could put then together if not the guy I met who reconed my subs said he would help me. He is who told me about making custom subs and that it's so much cheaper to make ur own. He said they will be like 2500 to 3000 RMS easy and that they will get super loud and low. Just curious if this is something most of y'all do or is this kinda of like an odd thing to do?

He said if I make these 21"s and do a huge 4th that I could get to 160dB probably with a good enough amp. His single 18" that he made with a 100lbs quad stack motor takes 3000w at .5 ohm full tilt all day. He gave me a lil demo and it was definitely alot louder than my 2 18s. But his was in a 4th