The least expensive route to a NE rifle is a Ruger Number 1. The Tropical model, the 1H, can be rebarrelled to .577 NE. I'd suggest the .500 3" instead, though. Much easier to shoot and components will be a lot cheaper as well. Handloading is the only way you can afford to shoot the big guns. 50 caliber hard cast bullets are available from Beartooth Bullets, among others.

I have a .500 in a No1, and it's a lot of fun.

You could also rechamber a 458 Win to .450NE for even less money. Use the savings to feed it!

For even less money, you could buy a factory chambered No1 in 450-400 3" and start shooting.

SSK Industries in Ohio does lots of custom stuff with these rifles.

Food for thought...