Ever heard of the "project triangle"? It's something a lot of engineers will be familiar with. Normally it demonstrates that you can have two out of three things, and any two, but never all three. The typical example is "fast, good and cheap". Any two of these can overlap, but it's not one of those Venn diagrams where all three overlap in the centre.
I've often had to hire subcontractors in my career, and as an engineer by training, I've always kept the project triangle at the forefront of my deliberations. I know that nobody can offer "fast, good and cheap" service. Working as a consultant nowadays, I try to work out which two of these a client wants, even if they don't explicitly say so, and deliver accordingly. I believe it's something we should all consider, whether offering or buying services.
More reading: Project Triangle at Wikipedia
What do you think?