A "bonus" is generally something given by the employeer when they recognize the value of something that came out better than expected. Harder for you than you expected isn't generally the criteria used for this term. Everyone has a hard day - that doesn't mean that it's above and beyond the call of duty.
What is an aspect of many short term contract jobs is reworking a change in scope. Contracting of many homeowner projects follows this model - your plumber, electrician, general contractor or other contractor will review the work that needs to be done and quote you a price factoring in whatever metrics that particular person/company uses for such things. It often includes:
- price of equipment
- estimated time to execute
- difficulty of work
Difficulty of work is often the real selling point - I see everything from "this is really hard, and I'm the only guy in town that can do this type of thing", to "this isn't hard, but because of the factors involved it will be EXTREMELY annoying... someone else might do it for cheaper, but I hate this work enough to not care if I price myself out of the market". They are generally not that blunt, but it's not hard to figure it out.
I use the metaphor of home contracting, because there's a real case that when they start work, and really SEE the problem, the problem will change. Most home projects involve opening wall board, or otherwise deconstructing the space in a way that would be inappropriate for an estimate. So there's a real chance that in the first 30-60 minutes, your contractor will be talking to you again about how the price has changed.
The key is that the scope of work is clear enough, that if it changes midstream, there is a basis for changing the price. That way there's a way to handle the "this just got harder" problem.
Typically, simply having to use a wider/better skill set is not grounds for raising the price midstream - presumably you have already factored that in, and can typically demand a certain rate based on skills you've already shown that you have. I have, for example, a plumber who is wonderful - anything he's fixed has stayed fixed, and I know exactly what he did and why so I can make better home decisions in the long run. That expertise is valuable, and I won't just hire some guy from Craig's list, even if that's going to save me 50% of my costs.