is quite relative. Some folks worry about every $10 because they have to or want to and others spend a few thousand without thinking much about it.
That having been said, G gauge is not cheaper than O gauge in general if you check out comparable prices. Command control is significantly more expensive in G. If you check out LGB diesels they cost pretty much what Lionel or MTH diesels cost. Similar injection molded plastic and sound, but no command in the LGB. If you compare the cheaper diesels from Aristocraft and Bachmann in G to Williams, you’ll find they’re similar in price. Rolling stock tends, if anything to be more expensive in G. Diecast steam locos are more expensive than injection molded steamers, so LGB steamers are a little less expensive perhaps than Lionel or MTH, but then again, no command control is built. So I don’t think G is much cheaper than corresponding stuff in O gauge.