I wanted to try out the 450BM but was on a budget. I already had a number of AR lowers and enough parts minus the barrel, gas block/tube, bolt and muzzle comp to put an upper together.
I ordered a barrel from Bear Creek last Oct for $74. Not sure what they cost these days.
I bought the gas block/tube from Omega for $14 and the muzzle comp from them as well for $18.
I already had a stripped Anderson Sporter upper (no forward assist or port cover) from Aim Surplus which I bought for $40.
I also already had a 10" free float handguard which I paid $38 for. BCA has about the same thing for the same price.
I already had a number of AR bolt carriers but the bolt head is unique to the 450/458. It took me about 9 months to find just the bolt head. They were $49.99 + $9 shipping at Surplus Ammo or $59.99 + $5 shipping at Sanders, so about $65 total. They weren't in stock at Surplus Ammo until after I ordered from Sanders. Oh well.
I mounted a 2-7x Vortex Crossfire scope $130 from Amazon.
I got a couple 450 follower off a friend for free but they are $8/ea on ebay.
Total build if I had paid for all the parts would come to $395 not counting the A2 lower I already had. It would have been more if I had to buy a carrier and bolt handle.
It shoots 1-2 MOA so I'm pretty happy with it. I had to open up the ejection port with a dremel to get reliable ejection but it is reliable and accurate.