I wouldn't call them Surplus that refers to old military weapons.
They are very affordable right now. The prices have come down so much that you can buy an entry level AR for what it would cost you to build one. But then building basic ARs does get boring.
But then look at how much has changed in the AR market over just the last four years. There are so many different handguards and stocks to chose from now. Then there is the handrail. Just a few years ago the thing to have was a quad rail. Then M-Lok and Key-Mod came out. Then the slim and ultra light rails pushed out the fat ones.
Today where you really save your money is when building a higher end AR. An upper end AR that will run you $1100 to $2200 can be built for $600 to $1200.
Then there are Theme builds. A buddy of mine did a Dead Pool build that turned out pretty good. I have a Sharps Brothers Hellbreaker lower that I plan to do a WWII P40 Warhawk build with.
I have already built a few 9mm, carbines and pistols. I built a .277 Wolverine and plan on doing a 458 Socom.
If you already have a few ARs and you don't feel the need for another, maybe one of the ones you have could use a face lift or some upgrades.
One of the funnest build I have done, I got a lot of help from members here on the forum. I wanted to build a gun from used parts, a Junk Yard Build. I ended up with with a super cool little carbine that is fun to shoot.
You could also build them with your kids. Here’s my youngest and his first build. He was 11 yo.
He later dressed it up in zombie camo.
Here’s his second build at age 14.
Building ARs can get boring, but can also still be fun, just use your imagination.