I am a professional commercial photographer with my own studio, not in London, but close enough to be viable and with a direct rail service. I'm sorry but this is not a $250 job. My studio and I are, however, free on Monday.
This is my calculation:
The studio will be out of commission for all other work for 24 hours: £120
Protective sheeting, installation, materials, removal and clean-up: £125
Four hour shoot, my fee: £280
Assistant on the shoot: £100
Total £625, convert to US Dollars: $800
This is shot using ordinary flour, not that much, but it does spread. Your model will have to know exactly what he or she is doing and take direction very well. Please provide the model's name and a link to his/her online modelling profile. A beginner will not suffice.
You don't use slow-motion. There is a technique for this type of job.
I shoot on white non-reflective vinyl (because it's much easier to clean) and back-light it for contrast taking it (most of the way) to black in the camera and then editing it to black.
I will need to discuss full details with you on chat here before going ahead.
You can save £125 by providing the plastic sheeting and installing it yourself. Please let me know if you'd like to do that.
Sorry if this is not what you wanted to hear but this is the right way to do it and the right price. Please consider reissuing the job in British Pounds Sterling.
I await your message
Kind regards
Barney Douglas
Hangleton Lane Studio