Most small commissioned features cost around 100 EUR. However, cost is set per individual request. We work with fixed price agreements, and estimate the cost based on the required hours of work, and the risk of schedule overruns. So, depending on what you request, the price may be lower or higher. Contact hanwen@lilypond.org for an assessment.
For every paid hour of work, we have to put in several unpaid hours with fixing bugs, making releases, keeping up with email, and doing infrastructural coding. In addition, you are paying for our special skill and knowledge: we know everything there is to know about LilyPond's internals.
Comparable proprietary packages, like Finale or Sibelius retail for EUR 500, and then you only get a single-user license.
Of course, if you think that we are expensive, you can always contact MakeMusic or Sibelius, and ask them how much they charge for custom development on Finale or Sibelius. :-)