Its also worth reiterating that there are many different ways to pay for your software from Microsoft these days.
You need to speak to a specialist in MS licensing at a reputable supplier and look at spreading the total costs of your licensing over a period of time.
Also as someone has already pointed out, if you have 400 users but some/most of them hotdesk, you can signifcantly cut your costs by looking at licensing per device rather than per user.
Again, speak with a trained MS licensing redeller.
You need to speak to a specialist in MS licensing at a reputable supplier and look at spreading the total costs of your licensing over a period of time.
Also as someone has already pointed out, if you have 400 users but some/most of them hotdesk, you can signifcantly cut your costs by looking at licensing per device rather than per user.
Again, speak with a trained MS licensing redeller.