Rookie contracts are not that cheap. If Philly takes Wentz at 3, he's due probably $24MM, which is entirely guaranteed. So now you have $50MM in GUARANTEED MONEY to your QBs. And Philly has some cap space but not a ton and they have some big contracts coming up that they have to deal with - Fletcher Cox, for example. And with what top DTs are getting in free agency, he's going to get paid quite a bit.
Miami made a bad deal. That was a total gift to get from 13 to 8. To give you an idea of what it might cost to move up............... the 3rd pick is worth 2,200 on the value chart. The 8th pick is worth 1,400. Now while the chart isn't exact, it gives you an idea of the difference in value to move up.
For Philly to get to 3, they are talking next year's #1, maybe both their thirds this year (they don't have a #2 pick). They really don't have players to trade either, unless they do something crazy like trade a Lane Johnson to a team that needs an OT.