I am unfamiliar with the one's you mentioned, but the jist of it is that compared to the minimal upgrades that you can get away with for a typical AEG in order to get an excellent field gun ~ 400 fps (TN Barrel, upgraded piston, and upgraded spring) is around the $100 price range. Now depending on the base make of the AEG, you may have (want) to do other upgrades such as switching out plastic bushings for metal bushings, or the aluminum piston head for a poly carb one in order to help the AEG handle the strain from the higher fps (with out cracking the mech box), which could place the cost up another $30-$80 depending on what you swap out.
AEG upgrade parts are manufactured by the metric ton around the world, and there fore competitive pricing keeps things reasonable.
On the other hand, BA upgrade parts are (in comparison) a very small market, and so the manufacturers have to charge more (to maintain a profit that they would normally get for the volume of sales) for their parts. There are also fewer (reputable) manufacturers of BA upgrades, and so there again is a premium on the price.
The average price of an AEG spring is $10 compared to $35 for a BA spring.
Plus with a BA upgrade, there is no half measures. You cannot upgrade the spring with out upgrading the piston and spring guide, otherwise you will be very shortly. A typical upgrade "package" (complete for various fps) can run you anywhere from $180 to over $300, not including a TN barrel, or zero trigger (which I personally do not condone). oh, and if you want to upgrade your barrel to a TN, you have to buy an AEG barrel of the desired length, which requires you to purchase an aftermarket hop up unit which will cost you and additional $65-$75 (not including the price of the TN barrel).
So I suspect that the models you posted with the $1000 plus price tags already have many of the upgrade options as a standard in their design. If money is no biggie for you, check out the Smokey's Barret airsoft (.50 cal) "sniper" rifle.......google is your friend.