When they talk about "the budget" they're taking about the face value of players, not the headroom for amortised costs. So the £100m relates to the total purchase price of those players. Otherwise our budget would essentially be £500m this summer, which obviously isn't right.
I’m not sure this is the full story. There are at least two constraints- firstly the amount of actual money we have to spend and secondly, FFP. Neither are directly related to the face value of players we might buy, as far as I understand.
The “transfer budget” is really an estimate of the sum of all the player purchases at their “face value” but will, in practice, be very elastic depending on how the fees can be spread.
The FFP constraints are complicated but don’t look at the “face values”, it is money “out” and money “in” and depends how the accounting is structured. Over a long period it really is just adding up the “face values” compared with income generated but in any one year or over short periods of a few years, the accounting is relevant.