Thirty feet is easily spannable with the simpler trusses, so cost needn't be extreme, this of course, needs be measured against your expectations and how realistic they are.
You have three options ---
Treated trusswork & deck open to the weather(potentially prohibited over a waterway) & [unplesant,unhealthy to cut and therefore more expensive to join]
Boarded in "Pony" trusses. Once common but no longer the image that comes to mind when thinking timber bridge ( best planned with a treated deck)
A traditional covered span (adding the expense of a roof system & roofng, but eliminating the need for treated lumber > except for the bed timbers, yet still no leaching < nor will fireproofing treatments be weatherworn)
--- You must add to the cost of the bridge ---
It's abutments (though if spanning a true ravine, bedrock should ~?~ be at or near grade, again minimizing cost)
Local Engineering requirements.
Good luck in your research.