Edge rates these days mean that my default starting position is 4 layers (Two layers is only for almost literally DC power and smallish audio things and even then....).
Buried via is insanely expensive, sometimes you have to, but on anything you can reasonably route in 4 layers you should be able to avoid it.
4 Layer is pretty much the minimum for acceptable performance if the board is even slightly non trivial, and should not add that much to the cost if you are doing a reasonable sized run (There is rather more setup involved).
If you want sane prices at small volumes then you must make sure the options you select match the board house defaults for a 4 layer, but for RF you will want to use someone who specifies the layer stackup so that you can calculate the track widths for the appropriate impedances.
I can and have done two layer RF boards (One routing layer plus ground plane on the back, the trouble is if you do this with a 1.6mm FR4 board, a 50 ohm trace becomes insanely huge, and even a 1mm or 0.8mm thick board does not really solve this, a buried ground plane say 0.2mm under the outside routing layer however, and all of a sudden your RF tracking is a sane size.
The ground plane is kind of non optional IMHO so a two layer stackup only has a single routing layer, where a 4 layer has two (And you can sometimes sneak some routing onto the power plane remembering that signals on the back of the board will be using the power plane as a reference so chopping it up is not a great plan, so effectively two and a bit).
Regards, Dan.