Step 1) Research and report on (a phpbb board currently live with 2500 users) current state of SEO regardling Google. Advise on improvements, mods, that you think might help
Step 2) Implement your advice and look at results before and after
Not really sure on the dollar value here so use my 250-750 as a guide only
Update: The above 3 parargraphs give you some background the real job is this
Install SEO mod or mods to a PHPBB3 forum to have the forum properly indexed by Google, modify / tweak forum to improve Google results.