It takes, 0.1 supply per day to feed 6 men supplies costing 6 credits each, 1 credit can feed a man for 10 days
Actually supplies are worth 5 (you just buy at x1.2 price, likewise you sell at x0.8 ), and my 800 man crew need 8 supplies per day. One supply can supply a man for one hundred days, therefore one credit is 20 days of supply.
Now the USDA[1] lists the average food budget of a man aged 19-50 to be between $41.70-$82.80 per week. If the crew members eat as cheaply as possible the amount per twenty days is $119.14. However very little farming was completed in the sector before the collapse[2], if we assume is ten times as expensive as a result then cost of food will be $1191.43. For simplicity's sake I will round these numbers to $120 and $1200 respectively.
The United States use 12,914 kilowatt-hours per capita annually[3]. The estimated total system levelized cost of advanced nuclear power in in 2017 is $112.70 per megawatt-hour[4]. This means the average US citizen spends $1455.41 per year, or $79.75 per 20 days. If we assume that the increased energy costs of a spaceship is equally countered by increased efficiency of whatever futuristic reactors they use, then the energy costs of a crew member can be rounded to $80 per 20 days. If we assume the cost of power is ten times as much (say, for example, the grav generators needs a metric crapton of power, or the reactor runs on some expensive rare material) than the cost is $800.
The average consumer expenditures on apparel and services was $1700 in 2010[5]. Since it doesn't specify what these "and services" are I'm going to assume for the sake of this calculation it's just clothing and the money that would that would be spent on "and services" are factored into the costs of whatever special suits the crew use at work. Hence, clothing costs are $93.15 for 20 days. From the same article, average healthcare costs from 2010 is $3157, which for 20 days would be $172.99. Again from the same article "other" costs are $3379 for 2010, which will be to $185.15 for 20 days. Adding these costs together totals $451.29, which we will round to $450.
I will not include water prices, as I will assume it is recycled by on-ship systems (therefore factoring into electricity costs) or included alongside food. Water isn't exactly a scarce resource in space at any rate so I imagine the cost to be relatively negligible. Likewise I will not include gas costs as I assume that gas isn't used anymore. Entertainment costs will be either a personal expense or be included in electricity (which suddenly makes me wonder what video games they have in the future. Earthfarer, perhaps?).
Thus the value of one domain credit is between $650 (if everything costs the same as it does now) and $2450 (if we dramatically increase prices to account for the setting). A vigilance class frigate is worth 5000 domain credits, in dollars that would be between $3,250,000 and $12,250,000. Considering that we are talking about a military spaceship and that an F-15E, a mere atmosphere based fighter, costs $31,100,000[6] either these spaceships are extraordinarily cheap or there is something highly expensive about supplies I failed to consider.
CITATIONS:
[1]
http://www.cnpp.usda.gov/Publications/FoodPlans/2012/CostofFoodMay2012.pdf[2]
http://fractalsoftworks.com/category/lore-2/[3]From Google, who got it from World Bank.
[4]
http://www.eia.gov/forecasts/aeo/electricity_generation.cfm (I would use the 2012 costs, except opening the associated PDF file keeps crashing my internet for some reason)
[5]
http://www.bls.gov/news.release/cesan.nr0.htm[6]
http://www.af.mil/information/factsheets/factsheet.asp?id=102