I'm of the opinion that if you can't afford the ammo- don't buy the gun!
No need to spend money to change the barrel on the .30 when you have two 9mm lugers already,
just shoot them and the .30 occasionally- or get rid of it.
Changing the barrel is probably a $200 excursion, then you have a mismatched 9mm luger, and a .30 bbl that is worthless.
There is no "cheap" ammo anymore, .30 luger or other. If you count anything for your time, reloading is not a great bargain either- but it is less $$ out of pocket, once you have the loader and dies.