Sky is the limit. You can spend $600 on a working 1990 yamaha exciter with pogo stick suspension up to $24k on a new mountain sled. All depends on what you want out of it.

As Schuerman says, one of the most EXPENSIVE cost per mile of any of the "toys".  If you are not handy with small engines you are best off buying what has the best dealer support.  I've been around sleds since I was 8 and started with a 1972 GP292B.  Largest pile of junk I ever owned/owned.  It made me a great mechanic fast.

I feel bang for the buck late 90's early 2000's were about as reliable and cheap as they get for "farm sleds"  I have over 20 miles of trails on our farm and the boys will spend HOURS riding around.  I pay for all gas and fuel and they pay for repairs.  Sure beats them sitting on their iPads like their friends do.  I would say they ride at least 3 times a week.

10 yo has a 1996 Polaris XLT 600 H.O.  Pretty good fit (he's 145 lbs and 5'4").  Price a two years ago you can pick one up with decent track and running for $600 all day.  Today, I see them on FB for $1200-$1400 and up to $2k which is insane for a 26 year old sled.

8 yo has a 1996 Skidoo Formula SLS 500 liquid with electric start.  Paid $1200 for it with 3k miles.  Nice and nimble sled and does decent in powder.  This is probably my favorite "farm" sled because it's light and has electric start.

Dad and Mom have a pair of 2011 Yamaha venture lites.  2 up and 4 stroke 500cc.  It has electric start, reverse, 144" 1.25" track. Pretty much goes where I need it to but just not real fast.  It does have gobs of torque for a 500. Has a nice basket/rack on the back to throw the fishing gear on.  Hand warmers and super easy on fuel. It's a boat but our youngest (6 yo) goes 2 up with me at least 75% of the time.  That's part of the fun.  FWIW, four strokes can get a ton miles without rebuilding.  I have 13k on one and 12k on the other. Run like the day they were new and take no oil.  Downside is they are incredibly complex machines (dry sump, outboard planetary to the clutch, 8 million sensors, etc.)

Late 90's early 2k I feel like Polaris 600 and 700 twins were the pinnacle of reliability.  Yamaha 600 and 700 triples were the very close second.  Early 90's the non RAVE rotary port Rotax were the "standard" for power and reliability.

Buying used it's all about condition condition condition.  Bad or worn track?  Pass.  Studded track?  Pass.  Motor knock?  Pass. Handlebar risers?  Pass.  Recently rebuilt "top end"?  Pass.  Tear in the seat?  Possibly pass ($150 for a seat cover these days).  It's just not worth doing anything major to the older ones because they will sell for more as a "parts sled" than fixing it back to workable condition.

I have a REAL hard time spending $24k on something that is designed to be dragged/pushed through the snow and used at most 3 months out of the year.  But some guys blow it at the casino so who am I to tell them how to spend their money :)

I could either way on EFI vs carb.  Poorly maintained carbs are playing with fire (lean condition and burn down).  Fuel injection is more tolerant of poor maintenance but can get spendy if you do a REALLY poor job with fuel (ie water in fuel).

Whatever you do, buy a cover and keep them shedded when not in use with the track off the ground.  Ground squirrels love to make nests and fill them with acorns.  Mice do what mice do.