You need to specify tolerances for you board before you can have any meaningful answer. What is your smallest trace width? What is your smallest spacing, smallest plated or non-plated hole? That aside, I often use 4pcb.com (Advanced Circuits). They have specials for 2 and 4 layer boards. For four layer board it is $66 per board and you need to order 4 min. It doesn't sound very good, but if you have a board that is relatively small you can create an array of boards and then cut them yourself. They will charge you an extra $50 for multiple parts but even so, if you can fit around 10 boards per array it puts you at ~$7 per board which is ok.
Here are the specs for that special:
Min. Qty 4 Boards, Lead Time 5 Days, 4-Layers, FR-4, 0.062" 1 oz. cu plate, Lead FREE Solder Finish, Min. 0.006" line/space, Min. 0.015" hole size, Plated or Non-plated holes (if no specifications provided, holes will be plated), Maximum 35 drilled holes per sq. inch, White Legend (1 or 2 sides), 1 Part Number per Order (extra $50 charge for multiple parts or step & repeat), Max. Size 30 sq. inches, No Slots (or overlapping drill hits), No Internal Routing (cutouts), No Scoring, tab rout, or drilled hole board separations, Routed to overall dimensions, Green LPI Mask
If you can get away with a two layer board you can cut the price roughly by half.
If you tolerances are tighter than those stated above, check out pcbcart.com I had a decent experience with them.