Pricing all depends on your area and clients.
there are members here who sell 18"X24" coro yard 
signs for $4 each ($1.33 ft).  depending on the job we range anywhere from $1.5-$4 sq ft.
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as for printing on lexan, is it white or clear?  If it is clear you will need a flatbed than can print white inks also.  if you have to under-print the white your print times will greatly decrease.
based on the quantity of boards you're talking about, you're going to need a seriously quick flatbed to print that volume each month (or run 3 shifts).
assuming you size and qty is correct you're looking at roughly 50,000 sq ft per month.  most midrange flatbeds (think $250,000) are going to give you good output at a 240-400 sq ft per hour.  That's over 200 hours easy, plus loading times, maintenance, etc.
Now, if you want to spend some serious coin (think $750,000+) then there are machines that will do what you want:
http://www.incadigital.com/products/Flatbed/Columbia_Turbowhite.php
hope this helps clarify the situation a bit.  if you have any questions I'd be happy to share whatever knowledge I have.