You can read about added value from a business, financial or marketing or tax point of view elsewhere, beginning from Wikipedia. The perspective I want to take here is one of negotiation, or rather conflict resolution.
There are two fundamental styles of negotiation and two fundamental strategies. The styles are adversarial and collaborative, whereas the strategies are value sharing and value building. Styles are intuitive, strategies will perhaps need some explanation.
Value sharing is when you cut an existing pie, especially in a zero-sum game (e.g. 50/50, 80/20 etc.). Value building is when both parties work together to create more value and not necessarily divide in equally, which is not always possible. The emphasis is on generating as much value as possible for the two of you. Jointly, if your style is collaborative (you can theoretically build value in an adversarial style, but it doesn't make much sense).
The difference between value sharing and value building is typically illustrated with a textbook example wherein one of the negotiating parties need 2000 fruit pulps and the other 2000 fruit skins, and there are only 2000 such fruits total. If the parties fail to get to the root of their problem — by talking because how else? — then they'll end up with way supoptimal results such as a 1000/1000 (i.e. proverbial 50/50) split.
It does come down to the talking, which, in turn, basically comes down to the thought. You and your translation clients are not necessarily negotiating just the rates for the same old defined translation job, in an adversarial way. It's more of a give and take for both of you as long as you're at least ready to go there.
Why I mentioned conflict resolution is because this usually comes up when discussing ADR, Alternative Dispute Resolution, particularly negotiation or mediation. But it's not necessarily about a conflict per se. Rather, it's about reconciling interests that don't intuitively agree. Try to be creative. Collaborative value-building relationships probably work the best.
Example: You and an agency are negotiating your rate for the assigment, somewhat bitterly. You know you're good at it, and there they are trying to pay you a student's wage because, supposedly, they can't pay higher. You give them some arguments justifying the higher rate, which they use in negotiating with the end client and land the contract at a higher price. You get paid more, they get to serve their client and possibly make more markup.
The funny twist here is that the end client may very well be happy with the service and quality and even the quality-to-price ratio. In some cases prices which are not exorbitant but which can still be felt inspire confidence. 'Cheap meet is what dogs eat,' is an old Polish saying. People can appreciate qood service for little money, but serious prices probably find it easier to inspire serious confidence and serious treatment.