4 teams gave a limited itemisation of costs as an average
(Force India, Sauber, Caterham and Marussia/Manor), though car build was separated into different parts and in some cases combined with R&D (which wouldn't change for the number of cars).
I will halve the power system, tyres, freight and fuel/fluids cost (they were all customer teams).
I will take 2/5 off the chassis production costs. This is because small teams typically have 5 chassis (except if they have a crash magnet driving for them), including one crash test/general test car and 2 chassis per "race car" per season.
I will take 1/3 off gearbox/hydraulics (they have some R&D costs to meet because they typically design their hydraulic systems but not the gearboxes).
I will take 1/4 off electronics (these are mostly designed in-house, meaning R&D's basically the same no matter how many cars are involved, and I'm generously guessing production cost).
Salaries will be cut by 1/25 because the only staff likely to be able to be released would be the secondary pit crew (12 out of a minimum of 200 staff) and even that's not clear because everyone on the F1 travelling team does at least two jobs - their weekend one and their set-up/take-down one.
Travel and trackside facilities will be unchanged because the costs are essentially fixed by the travel team count and FIA fees. Windtunnel/CFD fees are R&D and also unchanged. IT, utilities/factory maintenance, HR and professional services are tied into the size of staff and facility, plus fixed fees because it's an F1 team, so don't change either.
This converts the figures to a hypothetical one-car team cost of:
Hybrid power system $14 million
Gearbox and hydraulics $3.67 million
Fuel and lubricants $0.75 million
Tires $0.9 million
Electronics $1.53 million
IT $3 million
Salaries $19.2 million
Travel and trackside facilities $12 million
Chassis production/manufacturing $10 million
Wind tunnel/CFD facilities $18.5 million
Utilities and factory maintenance $2 million
HR and professional services $1.5 million
Freight $2.5 million
TOTAL $89.55 million
You can make the costs reduce by a third by having a one-car team... ...but the most you could possibly get would be three-quarters the amount a two-car team gets (50% of general team payments are on championship position, the other 50% on individual results... ...for which two cars' positions are counted). A team unable to get into the top 10 would be limited to 50% of what a two-car team could get.
Add to that the fact that pay drivers are a major income item, meaning you'd lose a significant income stream if only using one car, and that non-trade sponsors are likelt to pay half as much for space on a one-car team, and the equation falls over very, very quickly.
Three-car team permissions for the back half of the grid (with all 3 cars allowed to score championship points) would probably work better.