I'm a modeler/rigger, so I generally don't do commission renders, so that is a bit different situation. But...

I don't charge hourly rates for most things I do. I charge based on my previous experience with similar items and how complicated/time consuming I predict the project to be.

I always ask a 50% deposit on the innitial agreed upon price, with the understanding that the final cost may be more than the innitial agreed upon price, and a rough estimate of how much more it may cost and a brief explanation for why (most people don't know what all goes into a project, so details are only provided if they ask. Most don't ask). 

Exclusive rights to the model are more expensive than non-exclusive rights. If the final amount isn't paid in full within a reasonable time frame after the project is completed (a month or so at most is more then reasonable outside of special circumstances), then the client forfeits their exclusive rights and any portion they've already paid towards it and I'll sell it on whichever marketplace that's appropriate for that item in order to recoup the remainder of my fees. (I don't have restrictions on what I will build for a client, anything goes as long as I'm able to create it). This protects me from being "shafted" as the OP stated, by people who change their minds mid-way or aren't happy with the final results. They get to see progress shots during the project's development, so they have plenty of opportunity to suggest changes during production. Any additional (major) changes after I'm done with the project will usually cost extra, depending on what it is. For minor changes I don't charge. For updates that make the original item function better I also don't charge, because that's on me and my skills/abilities, not the client. They don't get the final model/asset until they've paid for it in full. 

I also charge more for any reference/concept/pre-vis material I have to produce/track down myself. The more detailed and specific the instructions are, the more it saves the client in total costs.

Anything I have to buy for the project is paid for by the client up front with few exeptions. That includes special software. On the rare occasion that I have to buy an item/piece of software that the client can't provide, then it becomes a business expense and is tax deductible (at least in the US)

But most importantly, I tailor my agreements to each client. If it's a client that I work with on a regular basis, they aren't subject to the same 'rules' as someone I've never done work for and don't know anything about, would be. In other words, I make exceptions for repeat customers and long-term loyal clients because I know they'll have more work for me in the future if they like what I do for them now. They also get a lower rate than what I would charge a one-time client. Building a relationship with your clients is really the most important aspect of it all, and always good to tailor things around the client's needs and resources as much as you can. 

As your reputation grows and more people are requesting your services, you can charge more for each piece. That's pretty much a universal rule in any medium I think. 

What you charge is really up to you. There's not a lot of difference between a hobbyist and a professional when it comes to making money on your work. The only real difference is whether you're charging for it or not. If you're going to charge, then make it worth your time, unless its a project you're specifically interested in. I'm not a professional either, officially, but for small projects I can complete in a day or two, I generally ask $100 to $200. But that can go up as much as $1200 or more for a model if it's going to take several days to complete. More if it needs to be rigged. I also schedule different clients on different days, based on what work I'm doing for them. It is definitely a good idea to keep track of how many hours you're spending on each project and adjust your fees accordingly, but I don't follow the flat hourly rate because that can be off-putting to a potential client who thinks the sound of $25 to $30 per hour is too much (that is the average going rate for most CG freelancers btw. Maybe even a bit on the low end actually), which a lot of people will balk at that rate, not realizing that they may be paying much more than that at a lower hourly rate when all is said and done. 

 

~Shane