I actually support a range of paid mods, some are phenomenal ( such as RSS, URD, VRC, ASR) and some are decent (Assetto Drive - the lack of damage limits the emersion). However, there are others that are decent, but quite expensive for what you get (RTM), which is a single model that is sold for $10 and these I am less supportive of, as a result of cost. Now, with AD (Assetto Drive), I am completely supported throughout, in terms of updates, and I appreciate (including the other phenomenal mod teams) the efforts that have to go into producing good 3D models and support throughout the lifespan of their creation. If you have ever been a Technical illustrator, Draftsman or 3D modeller, you will understand the efforts that have to go into producing something that is of quality is not just building the model, but actually working on details. This is where you gather the quality. If any of you remember the game Ridge Racer, you will understand my point on detail (cars looked nice, but not quite realistic looking.)
Next, you have those working on the physics of the cars - and remember, these are the elements that have to be updated most - they will put in multiple hours to ensure that what you get is as true to life as it can be, essentially the contact of the car with the road (the pitch, roll, yaw, weight transfer) and overall feel. The rigging of the models, which also gives the emersion of moving suspension. These things are what we all love and come to expect of modding teams and a lot of the good teams work hard to produce this level of detail for us. Of course we'd all want a quality mod that is free. But remember, no one wants a Formula One car that feels as though it is Bambi on ice.
What I am trying to say, I guess, is that if the product is quality and of a reasonable price, then by all means support the modders. Some of these games, Assetto in particular, seems to thrive on having a modding community and if they happen to be diligent with their product, then why not support them the way you would with any other brand. I also understand that there are smaller modders that would love to get paid as well. However, they will have to build their brand (through quality) and develop a body of work (a mass of models) that will catch the eye of the consumer ( the sim drivers) and can be referred back to that will allow others to assess their work. This is what has happened with RSS and others such as VRC, ASR, URD and to a lesser extent AD. As I have stated before, I will have no qualms in supporting a modder or team that can produce quality. It is generally a small outlay, and in the case of URD (various prices) and AD ($10), you can purchase multiple models.
Apologies for the long winded comment, but some of us need to try building a model first and then understand it can be 'a labour of love', also that there is quite a bit of involvement for good quality delivery. In short, support where necessary.