The MxM laptops are veeery rare, and normally are expensive, also the MxM technology almost never worked and had a lot of problems that makes it very unreliable to upgrade a component. Even if you get a laptop you have posibilities that you could never upgrade it.
Stil, if you want it i seriouly doubt that you could find a proper i7 without MxM for less than 400$ not to talk to find one with MxM, probably you would need to triple the amount of money to find one.
With 400$ as limit the only options i see are to find one i7 one or to generations old (without a gamming Gpu ofc) or try to seek a second handed MxM one, but well for that price probably it will be deteriorated or have severe flaws.