Normally we do installments, and Cagliarli need the money badly
Our only competition is Milan, and they're broke. So realistically we should be able to fend them off easily. Rumours were they were offering disgustingly low amounts for Radja. So if we give them 5m per year for 3 years, with some 1-2M bonus depending on how well we do then we would probably get him
As for convincing Radja to pick us. We'll almost definitely give him more wages than Milan can/would. And I surely don't need to mention the footballing reasons why we're a more attractive option. But I will anyway.
Milan would give him more game time. But that game time will be all domestic, and he'll be expected to reinforce a terrible team. Sure, he wants to be picked for his NT but if anything the awful Milan team could drag him down as opposed to him coming to us and slotting in to a much better team which won't depend on him.
Milan are 2 or possibly 4 (with some good luck) matches away from saying goodbye to CL football for the next year (most likely more than that). Even the Europa league looks like wishful thinking currently for Milan. We are definitely the more attractive option so long as we rotate properly and give him a chance.
Some people may look at it as if he would be our '5th choice' midfielder. Sure, if you look at it like that it sounds bad. But realistically, whenever our midfield needs rested both he and Marchisio will get starts in place of Vidal, Pirlo and to a lesser extend Pogba.
He should find more than enough game time here, he will be finally competing for trophies and earning a much better wage.