Just to clarify, you can buy bulbs by themselves for your car. But the part that has the burn would require to change out the whole unit(s).
I assume you have halogen bulbs and not Litronics/xenons? What year is your car?
It is expensive to replace those. However, their quote could easily land you a new set of Litronics/xenons from an alternative vendor. If you have normal headlights with halogen bulbs, you can always find a set on ebay for $300 (I'm guessing) if you don't mind the amber color for your turn signals.
Assuming your car is a 986 (1997-2004)...
Here's a set starting at $200.
http://cgi.ebay.com/ebaymotors/Porsche-97-02-996-or-Boxster-Left-and-Right-Headlights_W0QQcmdZViewItemQQ_trkparmsZ72Q3a1071Q7 c39Q3a1Q7c66Q3a2Q7c65Q3a12Q7c240Q3a1318QQ_trksidZp 3286Q2ec0Q2em14QQhashZitem230280240716QQitemZ23028 0240716
Here's another for $300.
http://cgi.ebay.com/ebaymotors/Porsche-Boxster-Headlights_W0QQcmdZViewItemQQ_trkparmsZ72Q3a1071Q7 c39Q3a1Q7c66Q3a2Q7c65Q3a12Q7c240Q3a1318QQ_trksidZp 3286Q2ec0Q2em14QQhashZitem320286112313QQitemZ32028 6112313QQtimeoutZ1218736984656
Since you're new, let me warn you, stay away from the dealership when it comes to repairs.