That's up to the builder and what he/she can fit on the panel. There's some sweet GPS setups out there on the market. But in airplanes as in guns, good glass ain't cheap. Any actual aircraft owners care to chime in on care/feeding costs?
Buying glass for an experimental airplane is a lot cheaper than for a production aircraft. A friend of mine recently retrofitted his Bonanza with a G600 glass panel with synthetic vision. I have to admit, it is an awesome IFR airplane with that setup. It has XM weather and traffic avoidance installed, as well. He spent in excess of 50K with installation.
I have another friend that installed a Dynon glass setup in his experimental aircraft and the hardware was less than 4K. Basic stuff, but gives an artificial horizon, heading, airspeed and altitude tapes. If an instrument doesn't have to be FAA/PMA approved (as in installation in an experimental a/c), it is much cheaper. Combine that with a Garmin 496 for about 2k and you have a good VFR airplane. Getting an IFR installation is more expensive, plus you have to have an IFR pitot static test done to certify the altimeter and xpnder to IFR standards. That can run $750 for the test.
My Yak is VFR only, but the pitot static test that is done every 24 months runs about $450. Equating avionics to guns fits, you can spend as much as you want. Hope that adds something useful.