I noticed this leak yesterday. Any idea what it could be, and how much would it cost to fix?

 

looks like the valve cover; might need new gaskets

 

it will cost $10 and 1 hour of your time.

 

def time for valvecover seal and valve adjustment; dont wait too long. looks like its already creeping down to your exhaust. in rare cases if its caked up bad enough it can catch on fire o__o

 

FWIW: Very similar (rocker arm cover gasket minor seepage) on my 03 TLS. Replaced TB, rocker arm cover gaskets etc.. 4 years and 40K ago. Recently, noticed seepage, and found valve cover bolts not loose, but not as tight as they should be. Cleaned up seepage, tightened all bolts to spec, and seepage stopped. Don't know how or why these bolts became loose, as they were tightened to spec. when I installed them 40K ago!