If you are going to get a reciever/etc completely custom machined with all the CAD work/etc done from start to finish you are going to be looking down the barrel of about 3000$ After everything is said and done.
CNC machining time isn't cheap. 60+ an hour is not uncommon if it disrupts normal machine time/has complicated fixturing
The CAD guy, even cheap college level workers, are still going to charge 20+ an hour and they have to take it from concept to completion. It's not a small job. Unless you know 100% that your napkin drawings will work and are correct you can expect them to take a couple weeks of doing this.
Then there's materials, 6061 alu is pretty cheap. Magazines can be bent sheet metal (maybe, based on the design) as I don't think the AEG stuff will cut it.
So to put some perspective on it :
-Figure out how the magazine works
-Figure out how your trigger group works
-Figure out what kind of valve and how thats going to go
-Figure out if you're using goblin shells...and then figure out how you plan to do a custom extractor...
You see where I'm going with this.