For a sign that size, channel posts will never hold in or outta cement.
You'll need 4pcs 4" x 4" posts or better would be 6" x 4" with 2" x 6" stringers. You know the height out of the ground you need, so you'll have to figure the lengths into the equation. They should be in 36" holes with footers and cemented to keep that wind-load from taking that sign into outer space.
Don't know your costs of things or operations much less what you can do in-house or not, so you're pretty much on your own for figuring it out, so like maniac said..... but we'd be higher at around $2,500 to $3,500 installed.... permits not included.