Moshiri's wealth is estimated to be between £1.5-2.5bn. This won't all be cash, but instead Everton will be part of his assets.
Now, he's put in approximately £500m already, with the stadium costing another £500m+, so you're looking at between 40-66% of his supposed wealth.
If he sells the club at a huge loss, it could impact on funding for other assets etc., so he will want to ensure the losses are as little as possible - if any.
In terms of spending, it isn't as simple as splashing dough on it because we are hamstrung by our income vs expense ration (profit and loss).