5% is the "max" recorded in the entire UEFA zone and that's probably a club with "low" budget since Barcelona spend the most with 10m which represents around 2% of their budget. (It's probably Lens)
Again these figures never take into account players who're still essentially academy level but are earning healthy salaries. Periera, McNair, Blackett, Goss, Powell, Varela, Lingard, CBJ, Keane, Johnson etc will be costing the club the best part of £10m a year on their own, despite in any normal circumstances them being nowhere near ready for the first team. Add into that depreciation of assets, coaching staff wages, groundsman wages, catering staff, the teachers we employ, the six figure salaries of some reserve player's etc. Just a totting up procedure in your head would suggest somewhere around £15-25m
United have nothing to gain in that deal, unless if you are thinking that Benfica will sell us players on the cheap, in that case they have nothing to gain from it. As for the homegrown part the UEFA requires 4 players formed locally(in the club) and 4 players formed in the country, and if you don't produce them yourself you will be shafted by the clubs that produce them.
Both parties would have something to gain. The clubs we'd partner with would get a consistent yearly income, which for any business is far more preferable than a sporadic and occasional £25m payment if one of their players makes it (plus United would still pay a good fee if they did). They'd also have the benefit of a couple of years whereby they'd have a salary-free player playing for them whilst developing. United would be in constant correspondence with their staff and would have access to a much wider network of young players. We'd also be able to purchase far more youth players (and loan straight back) without having to worry about them being able to play in our reserve team. Likewise a 16-18 year old wouldn't potentially be ruined by failing to integrating into a new culture, a new language etc all at such a tender age. I bet there is a hell of lot of 16 year olds also that would love to join United but don't want to leave their Country, but by the time they're 20/21 they would either cost too much or are on the radar of another club they'd like to join. You may also be able to negotiate a somewhat set transfer structure whereby we pay them a figure up front, with bonus payments based on appearances/goals etc as well as % sell on fee's when they leave.
Naturally this would be a normal negotiation; the yearly fee and transfer structure would depend on the club. Obviously a club like Genk would probably jump at the chance for a couple of £m a year, whereas Benfica might be looking for £6-7m.
Again 4 players produced in England is easy; we'll always be buying (potentially) top quality English players like Smalling, Jones, Shaw, Rooney, Carrick etc. The other four player's formed locally we could just reduce the squad size by four. These players don't play anyway so it shouldn't make a difference.