Singlespeed freewheels cost about as much as 6-speed freewheels and unless you're putting on NOS XT freewheels everytime (Was there even a 6-sp XT? I can't remember) then you can even spend much more on singlespeed freewheels if you want.

Chains are mostly cheaper. $8-100 single speed vs. $15-40 for 6,7,8-speed.

No more derailers to buy so there's a bit of savings there.

Maintenance is a bit less. You don't have to cringe as much when your bike falls on drive side.

I'd rather ride fixed roadie 14.5 miles twice a day than geared or SS MTB but maybe that's just me.