If the rub is directly opposite the crankarm, I'd worry that you might have bent the bottom bracket spindle, or dislodged the crank?

I would definitely let your LBS look at it. As for prices, $30 looks more like the price for a 39T middle chainring (I'm not having any trouble finding them on Google ). Looks like 105-level 50-52T rings are more in the $50 range.