There are certainly lots of things you can do to keep the cost down, but in general, yes raising an HG/PG child will be more expensive than a non-gifted child.
We love backyard science, so rock collecting along the creek, pond dipping at the same creek, raising caterpillars of native butterfly species, backyard astronomy (sidewalk chalk and a long sidewalk works great for illustrating relative distances of the planets), etc. have been nearly free activities.
Certainly a good local library can help save in book costs.
Thrift stores are another good place for books, craft supplies, and random cool items (steel drum [no mallets] for only $16!).
Costs that can be hard to avoid: testing fees (IQ, ACT/SAT), instruments/lessons (if your kiddo is musically inclined), if the public schools are not ideal - homeschooling or private schooling have their associated cost/opportunity costs.
Best of luck,
-S.F.
