His value right now is around 14M.
If the bonus is easily attainable, being just a way to pay in more installments, it's an OK deal. His destiny was meant to be stg like Napoli. He's not good enough to attract foreign investment and would fit perfectly in clubs like Fiorentina and Lazio, which just doesn't spend this kind of money in one player. So Napoli would basically be the opportunity to sell the guy.
Also it looks like he's out of favour with Mihailovic. When deployed as a winger, he's not even above average. Sampdoria probably accepeted this because he's a bench player now and his price would probably fall after the season. And who knows if Napoli would be interested then. They need someone to replace Insigne now.
And his numbers might be inflated because he probably won't be the dead ball guy in bigger clubs anymore. Pretty much like Giovinco.
So I'd say this is not a bad sale. Not great either. Just OK.