The 50 BMGs are big bo doubt, thats a nessesity because of the size of the round and the recoil they generate. The more accurate 50s are rarely much more expensive then a top end full custom rifle
of conventional chambering. Several can be had for less then $4000.
I have owned several of them starting out with the LAR Grizzly and it was an extremely accurate rifle, just did not care for the shellholder bolt design.
Ended up with the Armalite AR-50. I really like the AR-50. Reasonably priced, with taylored loads tends to be very accurate but you need match grade handloads. Surplus ammo is fine for plinking but will not get you fine accuracy, its not made to.
One great aspect about the AR-50 is that its easily rebarreled, just like any conventional bolt action rifle, to get a new barrel put on or accurize the receiver, its no different then any other rifle. In fact my last AR-50, I rebarreled to my 510 AM with a Heavy Lilja fluted barrel. It was easily a 1/2 moa rifle at 1000 yards in good conditions when I was on top of my game.
I sold that rifle but just purchased another AR-50 and will rebarrel that one with a lighter, fluted sporter weight Lilja for a rifle in the 35 lb range, instead of 45 lbs that my last one scaled out at.
As far as cost in loading, lets be honest here. If you shoot a 338-378 Wby, your going to pay $50 for 20 brass and if you shoot a premium bullet, you will often pay $1 to $2 per bullet. Powder will be around 120 grains of powder depending on what powder you use.
You can get virgin brass for the 50 for around $1.5 to $2 each so its even cheaper then the Wby brass or Lapua for that matter. Another point, you will never wear out a 50 BMG case if you load it properly and anneal the necks every 5-6 firings.
The bullets do cost more for the 50 BMG. The 750 gr A-Max will run you from just under to just over $2 a piece depending where you get them.
Powder is alot more in volume but often about 1/2 the price if you use surplus powder.
Per round, yes they are more then most high intensity conventional chamberings but realize your producing at least 2 to 3 times more energy them most conventional chamberings, even the largest ones.
I once figured out my cost per round for my 50 BMG and for practice ammo, it came out to around $2 each using surplus(new) brass, surplus powder and surplus AP projos. This is not overly accurate ammo but it sure makes gravel at 600-800 yards!!!
My match ammo was around $5 each. Now this is including brass cost. Once that is paid for its much less then that.
You need dies and a press that will work with the big 50, that is easy, RCBS has packages for the 50 BMG loading as does Hornady. Both work fine for starting out.
Ballistically, the 50 BMG is not overly impressive as far as drop goes, in fact, a 190 gr SMK out of a 300 Win Mag will match the 50 BMGs trajectory nearly to the nuts. It does have alot less windage and a ton more energy, well, several tons more!!! LOL
Owning a 50 BMG is not all about ballistics, its mainly about owning a 1/2" gun and as mentioned, its just alot of fun. I have never had anyone come off one of my 50 cal rifles without having a big smile on their face after throwing 12-14,000 ft/lbs of energy down range!!!
For the money, the AR-50 is hard to beat.