They might have been 3m a week ago, but I've been seeing them 8-10 lately.
Infils are expensive because everything's expensive right now. Also they're really good. A +5 infil
- should equal or beat a +8 double bloody boned jur at anything below 120 str. A +7 infil
- beats a +10 2x1x jur. An infil  looks at a jur and laughs. Specialty jurs can beat infils, but only at high str.
Not to mention that even if you made a 2x1x jur, the skel worker card alone
would probably cost 8-10m. And the high atk of the infil makes it a decent PvM weapon anywhere you can't afford to make a carded jur for.
As for what to card an infil with, I'd go with boned. I don't remember whether RO considers infil's % damage to demi the same as hydra's or not, but I'd be willing to wager it does based on how other things work. If RO doesn't add them, boned beats bloody below 110 or 120 str because of the 5 atk. If RO does
add them, bloody does 170% damage and boned does 172.5% damage.
Edit: For clarity, infil
- is 150% while 2x1x jur is 161%, but the huge atk difference between upgrading a level 4 vs level 2 weapon means you need a lot of str for that extra 11% to win.