In 2005 I bought a 40' catamaran. It had no bridgedeck cabin, but it had a mast and some rigging equipment. Sort of a fleshed in interior, but no stove, a few lights etc. Basically a 40' daysailor.
In 4-1/2 years of part time work (~1.5-2 days/week) I added a bridgedeck cabin, built a new galley, installed a diesel, new genoa, radar, SSB radio, dinghy, watermaker, windlass, anchors, more sails, etc etc. Everything required for bluewater sailing. Sailed it around the world from 2009 to 2017.
It cost me 65K for the bare boat and 70K for the building/outfittting/ etc. etc. = 135K Canadian. Or in USD, very close to 100K.
I kept a very detailed spreadsheet of EVERY purchase for the boat. Down to nuts and bolts and sandpaper (god I spent a lot on sandpaper). Happy to send it to you if you want.
I would not have expected to get a used catamaran of comparable outfit for less than 175K USD or more at the time.
I am also very very good at buying on ebay, craigslist and had a wholesale account with a large marine distributor. I did not pay retail for very much (mostly fasteners, paint and sandpaper).
So... to be realistic, you need a study plan from a designer that has a materials list.
To that add all the equipment needed. You can use a big marine retailer like Defender and use retail prices for now, discounting say 75% for "good deals" that you might find.
To give you some very rough numbers on big ticket items
Mast/shrouds/boom - 20K
Main / genoa / spinnaker - 10-12K
Winches, running rigging, blocks, etc. - 5K
1 x 30 HP marine diesel engine - 15K installed by you with usual auxiliaries
watermaker - 2-5K
dinghy outboard - 1-3K
inflatable dinghy - 1-3K
stitch and glue GV10 dinghy
electrical system, batteries, lights, wiring, breaker panel, etc etc 3K
antifouling paint, primer 1K
anchor, windlass, chain, rope 3K
I do not think prices of boats should be the foot. Boats grow in 3 dimensions (width, depth,length) as you increase the length. Prices by the lb is a smarter measure.
Are you alone or do you have a family? A 40' cat is quite suitable for a family of 4. We had friends with 3 girls on a Lagoon 38. If you're single, a smaller cat say 35' is far more economical.
Do you have any experience sailing? Have you built a smaller boat? Or a house/cabin etc? A 40' catamaran is around 8000 to 10,000 man hours of work, or about 4-5 years of FULL TIME WORK. Depending on fit and finish levels.