@TsunamiMike You could dispense with beer gas all the way I suppose and some pubs do. But it changes the character of the beverage. Even the same stout if you had one tap running on CO2 and another set up with beer gas for the same stout ( different keg of course) they really would be a different experience. I've had some IPA on beer gas and was underwhelmed, blanketing the beer in thick foam doesn't let flavour or aroma flood out and the beer is much flatter. Those little bubbles of CO2 coming to the surface bring aroma as well as CO2 and they change the beer pH a bit.
The most cost effective way is to use CO2 for all the beers. As one regulator and then a manifold ( splitting the gas lines) to the kegs will work for all.
New Gas means new separate regulator, new cylinder separate lines, new tap. That's the budget.
You get more pours per CO2 cylinder than you do with beer gas.
@Twinkeelfool What pressure do you run in the Nitrogen at when you use it to serve?