If your spring has a reasonably close catalog equivalent then minimums run from $35 (for most smaller springs) to over $100 (for large stainless steel springs).
If your spring does not have a reasonably close catalog spring equivalent, then custom spring setup charges run several hundred dollars.
Does the spring cycle 10,000 rpm 24/7 360 days a year and it must last 20 years
or is it used/adjusted just once
or is it installed and then moves only a little bit
or .... whatever.
* Max Deflection:
What is the maximum deflection that the spring must accomplish without failing.
* Failure: What does "Failure" mean - in your application? That is a serious and important question.
Is weakening or breaking over time unacceptable. Or can the spring be replaced periodically.
IF the spring does not return to it's original position after being fully deflected is that a problem. Sometimes, it is not not.
or ..... whatever'
* Shock Loads:
Springs do not tolerate shock loads well. Applications such as firearm recoil springs generally fail much sooner than would be expected.
As with a paper clip, you can only bend a piece of wire so much, and then it won't go back to its original position (That's "Yielding").
Even if you don't go that far, but bend it back and forth too many times, then it can break (That's "Fatigue").
Stress calculations are a bit tricky. A spring calculator should be used.
Some simple rules of thumb are handy though.
Stresses are propertional to the wire diameter cubed (Making wire diameter critical).
Stresses are proportional to the length of the wire in the spring (making the OD,and number of coils important; but much less so.)
A small increase in wire diameter can make a spring much stiffer.
The longer the wire (more coils or bigger OD) then the weaker the spring.