I'll leave one last comment because the idea in general is quite intriguing.
The MOSFET you chose does look capable of what you want it to do - but only if you drive it well enough. That requires proper gate drivers.

100V to the high side is no small feat, I know because I dealt with these voltage ranges and switching FETs before. You can't get around some way of providing isolated power and an isolated data transfer, and without a good FET gate driver on each bridge FET they won't perform as expected and can fail, show higher Ron, switch slower, etc.

A good half-bridge driver can also take care of critical features such as some protection and switch deadtime, without which your circuit will probably self-destruct more frequently.

At these kinds of power levels, you will also almost definitely have to use current feedback to get a stable and reliable current loop, which means at least one proper current shunt, preferably two to measure the complete system currents.
Without them, your motor's current can go rogue if it experiences any kind of useful torque. These sensors alone cost a few extra euros, so again, leaving away a proper half-bridge or driver IC just doesn't save that much money.

Also - what motors are you going to drive that the price of the motor for this power range doesn't just outweigh the cost of a gate driver?
Even smaller 12V Hobby BLDC are in the upper 20€ range for powers, and anything above 50V will be quite a chonker (technical term, I know).

I'll leave a last recommendation for you to look into: The TMC6200

It's not up to 100V, it's only designed for 48V (60V max) systems, but it includes all the gate drivers, charge pumps, and even current sense circuits in one package.

If you want to design your own motor driver, this will be the best starting point. Anything above that will require more knowledge about high-voltage circuits, isolated gate drivers are a must-have, and the chances of blowing something up become higher too.

Good luck, and best regards,

  Xasin

Considering that you say that even just the 50ct per piece gate drivers will dramatically increase price, the FETs themselves can't be that expensive either - which means I doubt they will be able to carry a lot of current.

Keep in mind that most of the "rated current" is a bit of BS, you really need to look at the On-state resistance and do some thermal calculations to be safe, which can change how much current you can draw quite a lot!

P-Channel FET for low currents also shouldn't be that hard to get - or perhaps you could get away with BJTs instead of FETs, which might be workable?

In any case I highly agree with Florian Festi - there are tons of super cheap BLDC drivers, some even with integrated FETs and everything, massively reducing component count.

Look at the TMC6300 - 2V to 11V, 1.4A max phase current (RMS), with all the FET and charge pumps built-in.

The whole chip costs only 1.5€ - remember, that includes all six integrated MOSFET.

You would be wrong as I'm looking at the PSMN5R0 which can be had for $1.29 (in bulk) or $7.74 total.  Adding 50 driver means adding another $1.50 to the cost.  If I can save 50 cents, I would like to. :)

Well, there is nothing wrong with building your own BLDC. But I wonder why you think you can build one cheaper than the ones you can just buy. BLDCs are standard components that have been engineered to be cheap already - by people probably more capable than you. So if you want something cheaper you first need to decide which corners to cut that others didn't - and why this is OK for your use case.

To switch high side N channel MOSFETs you need some voltage that is high rail plus the switching voltage of the MOSFET. These drivers generate this voltage on their own e.g. by using a charge pump. If you want create a cheap solution you want only one of those for the whole BLDC and not for each half bridge. You still need a way to switch the gate between the plus rail and this even higher voltage - typically from electronics residing on ground. So there is a large voltage difference between those two parts of the circuit - that also depends on the motor voltage. This is not that easy and why those driver chips are a bit more expensive.

I know it seems a bit nutty but you'll never know what's possible unless you try. This is the MOSFET I'm driving: https://rocelec.widen.net/view/pdf/1jc1phhc8v/PHGLS23342-1.pdf

How much voltage?

100V(max) though 60V to 80V nominally but that's really beside the point.  Here's the MOSFET datasheet if it helps: https://rocelec.widen.net/view/pdf/1jc1phhc8v/PHGLS23342-1.pdf

If you're going to attempt an ultra cheap circuit, like capacitive coupling up to the high-side mosfet, then these sorts of details (and many others) are very much on point.  That type of analog circuity just barely works over a pretty narrow range of conditions and comes with a lot of pitfalls, so it needs to be carefully tweaked.  It takes a very careful & diligent frame of mind to consider so many analog details.  I'm sad to say if you feel the voltage is beside the point, odds are slim you'll ever get such a thing to work.