The way you should do that is lets say you wanna bid $3- for your mobile keyword, you will have to bid $1- for the desktop keyword and set the mobile percentage to 300%, that is the only way, cus Google needs money that is why they took out the mobile only option and have tablets put under laptops.
It is really hgetting harder and harder to make money with these motherfuckers, you're better of searching for other sources of traffic.