Hi OP, mid main here.
I've been playing mid since oh... beginning of S3 or so (damn, that sounds so much shorter, Riot Addiction OP). Even back when the supports had 2837458734895 wards in their inventory to ward with, I still warded mid, because you shouldn't be relying on your team to keep you safe, so bravo for considering ways to adjust to other players not doing what they're supposed to.
That being said, that item slot loss hurts magi alot. Unless you're playing say lee sin, etc, you're probably not going to want to pick up a sightstone mid.
2 situations : Either you're face rolling as mid, and you have lots of money to grab wards every back, OR
You're losing mid, and you've only got barely enough to get wards and pots every back.
Either way, here's the rule of thumb I tell -every lane-
If you're leaving the fountain > 75g left and you have open item slots, you better have a ward.
Also! Don't forget to place and replace your pink throughout the game! If the enemy doesn't find it, that's a long standing ward for only 25g more!
(And yes, I do expect ADC's to ward, I do when I ADC, and it often saves my life).
But really, I think it's more efficient for mid to simply get wards each back. If you're roaming properly, you'll be getting plenty of kills and assists so you'll have no problem picking them up. =)
-T