It's been a while since I bought trims, but I remember a huge price range for a variety of trims that looked pretty similar. I have always gone for the cheap end of the range as long as I was satisfied with the appearance. I have never had any problems with any of these trims discoloring or deteriorating. A trim is not a high tech, sophisticated object and they are manufactured for pennies. Shop on the basis of price, and preferably get them somewhere you can physically examine one to be satisfied you like the looks and that it does not seem flimsy. Another advantage of buying locally is that you could easily return them if you weren't satisfied with the look once they were installed. I'd be completely comfortable buying these from a big box store.

We wanted the same thing -- plain white, nothing fancy. I think we paid less than $5 each. This is the second house we've built, and we used the same trim on the can lights in both houses. Haven't had to replace one yet. We actually got them at a local lighting specialty shop. The big box stores were more.

Recessed lighting fixtures consist of a housing (installed before the ceiling) and a trim kit (installed after the ceiling). If the contractor is going to provide the housings you will be limited to that manufacturer's trim kits. Many trim kits will only fit in certain model housings so the design should be determined before anything is bought. It would seem wiser for you to choose the trims now and have the contractor buy and install them.

6" recessed lights save money because the lamps can have a larger wattage and larger light pattern so they can be farther apart. The downside is that they often look large, throw more shadows, and the lamps are very visible (especially with R-lamps). Consider adding a few fixtures, placing them closer together, and making them a smaller diameter.

"A trim is not a high tech, sophisticated object and they are manufactured for pennies."

Some trims are cast aluminum, some are plastic. Some trims provide an air or water seal. Some trims are highly decorative and very expensive. It would be difficult to generalize about the cost of recessed lighting trim kits but none of them are manufactured for pennies.

The link below is for a $183 Lightolier Calculite Evolution Trim Kit #C3MRWCLW that fits in one of their C3LV frame-in kits (housings).

Here is a link that might be useful: C3MRWCLW trim kit

Renovator8-
Yes, I know you can find expensive, sophisticated trims, and pay an arm and a leg, but that is not what most people install. In particular, the OP said: "We are not looking for anything fancy just standard white." That's what I responded to. The last time I looked at lighting fixtures, a large number of them were made in China and the standard trims do get pumped out for pennies. In my remodeling experience, lighting fixtures are one of the most over-priced items in the building trade.

I guess what I should have said is that recessed fixtures should be selected for their location and function first and then the trim options should be considered. Often there are few options. As for the part of the trim kit that is exposed, white plastic will discolor; but white cast aluminum will not so you usually get what you pay for.

If the OP could explain the function, brand and model of the fixtures offered by the contractor as well as the ceiling height and what is above the ceiling, we could offer better advice about trim kit options.

In no case would I recommend buying a recessed fixture housing or trim from a box store. Even the quality of the lamps is poor. The cost of replacement lamps can sometimes exceed the cost of the fixture in the first year.