There is no true "rule of thumb" on costs, but normal residential systems commonly work out to about $100 per head (including piping and controller) depending on complexity of layout and digging difficulty (can up to double cost), or about $500/zone.
In your case that would give (assuming 2 zones for the mulch area because 12 heads is way too much for one zone) about $8-10,000 - but your system is much larger than normal house, so your per-unit cost would go down, particularly as it is large enough to justify bringing in a trencher to lay the tubing.
I would question whyu 12 heads for a mulch zone - is there any change one or two flat sprinkler strip hoses could do the same job for a lot less cost - unless quite wide ?
So - I would say certainly not overpriced - I have seen much smaller systems go for that price, so either this is a low bid or there is a lot of competition in your area.
I presumefor this size job you got 3 bids - where were your other bids - if way higher but close to each other, time to worry about whether this guy underbid or not. Also, be sure you are talking firm BIDS, not just ESTIMATES, which some guys lowball to get the job and then crank the amount up later on.