I provide phpBB services and have two years experience.
There's nothing much to say,so here are the prices:

Installing MODs - 5-10 dollars. depending on the level and time.
Installing styles - Free
Fixing any errors/issues - $5 for 30 minutes.
Creating a custom MOD/Style - depends on your request
Conversions from any software to phpBB - $5
Update your phpBB board to latest version - $5

For now,I receive payments via PayPal only.
Before I start,the webmaster have to provide me with FTP details and a founder account on the board. and %50 of the price must be paid before starting the work, just to make sure that everything will go fine..
PM me or post here for more info