35 61030019050 CIRCLIP OIL LEVEL GLASS 03 $0.41
36 59030019000 OIL LEVEL GLASS G 1777 Y $14.65
Parts ARE available separately, your dealer is misinformed(or trying to sell you something you don't need, but that would never happen, right?)
If I remember correctly the window is mounted in its own seal and the circlip holds it in the cover. Once you have the clutch cover off it should only take 10 min or so to replace the window. KTM recommends some Loctite 243 on the seal (find a parts microfiche).
Good luck! nick