- cost to provide, or harm or inconvenience sustained on the part of that someone (similar to cost plus pricing)
- benefit created for that someone (value-based pricing, which does not necessarily involve manipulation of supply and demand)
Your costs are not only the bills you pay and the prices of equipment and software you use but also all expenditure, loss, detriment, inconvenience etc. suffered on behalf of your client, as well as opportunity costs, which means anything that you sacrifice in order to satisfy your client's needs or desires.
- lack of remaining capacity for other work, sometimes also on the day after (because you will be too tired to work — a small surcharge doesn't make it even)
- consequences of unavailability to your other current or prospective clients (new relationships not formed, existing relationships strained, substitute translators taking over)
- increased fatigue, exhaustion, loss of sleep, which, apart from being unpleasant, you just can't afford to repeat too frequently for fear of more serious complications
- loss of family time or time alone or private opportunities, including many things that don't even have a price tag on them (what's the price of not putting your child to sleep)
- sheer inconvenience of dropping everything and upsetting your plans
Your client's benefits include:
- ability — or increased ability or capacity — to reap the benefits of short-term opportunities by responding timely to them (e.g. time-limited bargains, RFPs/tenders with tight deadlines, certain types of commercial transactions)
- stronger image and better PR because of acting first or responding in time, decisively and proactively
- competitive advantage over those who can't or won't
- ability to avoid negative consequences (including long-term) of all kinds (not just purely financial but also image and goodwill) of failing to meet deadlines and schedules or respond to opportunities
- sheer convenience of jumping the queue
As you probably realize, your client's benefits mostly relate to commercial transactions and investments of which the goal is profit, not charity.
Transactions involve transaction costs: the costs of searching, negotiating and enforcing, all of which can require translation assistance. Translation is also part of investments, or an investment in itself, hence translation cost is investment cost, which means the necessary cost paid now for a greater profit in the future (which will not be shared with you).
Bottom line: translation fees are the result of your client's profit-making operation.
You can't have a French website without translating it into French any more than you could have apple jam without planting apple trees — or paying someone else to plant them for you. You can't just spend your last savings on an expensive car, you need to be able to afford fuel and insurance. You can't just buy machines, you need people to operate them. You can't just buy a building for a one-off price, there's upkeep to pay forever after.
Having to pay for translation — or faster translation than would normally be available — is not any more unfair than having to pay the cost of seedlings for plants, fuel for cars, phone bills, electricity costs, salaries of employees and everything else, including any applicable urgency surcharges.
The expected profit from your client's business transactions is not a holy birthright or human right that you should drop everything to help secure. It's an investment that depends on initial costs, a normal commercial profit, not a humanitarian cause.
A for-profit business enterprise looks quite silly expecting return without investment.
It also looks silly expecting you to be a partner in investment costs without making you a partner in investment returns.
By the way, have you noticed that nowhere here did you and I consider supply and demand, only costs and benefits?
Not that there would be anything wrong in using supply and demand to avoid being underpaid (railroaded into 'best rates') by manipulation of supply and demand to begin with!