Would he? How much growth can he expect to make against the outlay required to buy the additional shares that'll realistically be at a premium?
He has the principal, obviously - he's a very rich man. SO if Arsenal only increases in value by 5%, he gets a nice big boost.
Only football is in the middle of an incredible bubble. A bubble funded by people like Usmanov, largely. So clubs worth are growing very quickly (honestly, I'm chatting bubbles here, I have no idea if it's true, but it seems likely and I don't feel like doing the legwork right now), especially marquee clubs like Arsenal.
He could easily expect the value of Arsenal to grow by hundreds of millions of £'s over the next decade. Buying 'low' would suit a man looking at profit, even if the 'low' price is £400million...assuming he expects to get £600m+ on those shares in 10 years...which isn't the worst bet.