It is a subjective question. The price of steel in your area, how far away the grinder house is, and how much they charge for your piece vs using their stock. All you can do is get estimates. Rates vary across the nation.
Blanchard ground is nice, but not a requirement.
Over the winter, a close friend wanted to build a new table, but since he knew I had a mill with a 11"x58" table in my garage, I let him rope me into machining the all the surfaces of some plates for him that were the same size as you are doing, at 6" x 36". The finish is as good or better than blanchard and I drilled and tapped a few holes in some of them for holdowns.
If you know anyone that has a large mill and are willing to pay them for dulled or broken cutting inserts, It is the cheapest way to go, but not many folks have a machine like that in their garage.
Well, He does owe me helping pull my truck axles this summer.
(edited for spelling)