If you REALLY want Metal Slug, you can find it quite easily for MVS. Just buy a converter and you should find one pretty soon.
Also, if you check out the selling section, I'm offering a converted cart for AES (no inserts, stickers, etc-just the game and black old-school box) for $205 (includes shipping). Quite a bit cheaper than $1300, no? <IMG SRC="smilies/wink.gif" border="0">