Nice Design.
My thoughts on your quote:
Overall I believe you got a good bid, however I think you could get that quote below $50k with a little extra work.
You could save ~$1000 off the top if you did the downspout connectors and drain pipe yourself. It isn't a hard job and all the stuff is available at most local hardware stores (4" pvc sewer drain, connectors, glue, and a shovel). Wet the ground a bit the night before and dig the trenches. Keep in mind that there will be a ~6" layer of compacted gravel and a 4" layer of rebar or fiber reinforced concrete deck on top so there's no need to dig the trenches any deeper than 6-8". This job could easily be done in a Saturday morning. Same thing for the sprinkler re-route. A trencher might cost you $80-100 to rent. Well worth it. But I certainly understand preferring to just pay someone else to do it. Note: Youtube has some great tutorials for home improvement projects like this.
The price of your arbor seems a bit steep. I paid less that half your quote for our 16x12 pavilion. (Finishing the project now, will post pix soon). My structure was built with pressure treated 6"x6" posts and not cedar. I'd be surprised if the $7k difference was purely due to the cedar but I'm admitedly ignorant on wood pricing. I recommend you check the yellow pages for local roofers or framers. I wouldn't necessarily use the pool builder in this case. Have you thought about wiring it for electrical? lighting? ceiling fan? tv? outdoor speakers? Even if you don't opt to install electric utilities, lay a few runs of conduit. The time to do it is before your decking is poured. The other thing I noticed is that it's spec'd at 15'x20' which is kind of wide to only have 4 support posts. Looks like you're in texas so wind loads have to be a consideration. I have no clue regarding building code requirements but at 12' wide (post-to-post) our builder was able to use standard length 2x12s for header beams and 2x8 cross-members. Lengths greater than 12' aren't very common. I'm wondering how it's going to be braced internally. Might be a good idea to get the arbor's dimensions in writing. Have them specify 15x20 outside post to outside post or roofline perimeter dimension. Big difference. Have 1 or 2 local roofers quote you a similar structure.
Again, these are just some things to consider. Good luck on your build!