Way above what I would ever pay for clothes, so these are just some thoughts about dealing with an artisan and small business person.
You are primarily concerned with quality, fit, and style, or so I assume. The challenge is finding someone who can deliver these for a price you are willing to pay. Haggling down to a discount but not getting what you want can convert a set of expensive bespoke pants into MTM.
There is nothing wrong with asking the price, but it may not be simple for the tailor to give you an answer. If you wanted multiple pairs of identical pants, perhaps. But as you change cloth the material cost will change for sure, and possibly the time and effort involved in making them. If you cannot tell the tailor exactly what you want, it may not be reasonable to expect him to commit to a price.
Obviously I am not a tailor, but under these circumstances I might be even more hesitant to make something when the customer is supplying the materials. The tailor has to stand behind the finished product, but now he or she has not been able to select the cloth. I suspect experienced tailors have cloth they like and cloth that, however nice it looks or feels, they know will perform poorly for a particular purpose. So they don't offer it. Maybe it does not work well during construction , does not drape or wear well, they don't trust the quality from the manufacturer... Seems crazy to pay a lot of money to an expert, then substitute your opinion for his on important details. Like commissioning an artist, then telling her what kind of paint to use.
On the other hand, there should be nothing wrong with telling the tailor the whole plan and asking the cost of the subsequent large order, if it comes. The answer may be "it depends", but you should be able to ask without offending.
The conversation will probably be smoother if you make it clear from the start that there is no maybe about the initial purchase. Do your homework with referrals, look at his work, and by the time you start using his time be sure you will pay for it. Think of it as hiring a consultant.
I work this way when I have custom things done, although I am a strict OTR guy when it comes to clothes. For me, the depreciation is far too high to go any other way.