As someone with limited experience of framing, what is - roughly speaking - the added cost of float mounting a print compared to a standard mount?

None really.

No added cost plus the benefit the whole print will remain the same colour (rather than seeing fading between the print in the light andunder the mount).

A decent framer should ensure the print has been flattened well prior to floating.

I notice a lot of people around here get their prints float framed. Is that just a stylistic preference or is it more of an archival option?

Also, is there some sort of guide to framing out there? I don’t really know anything about it and I tried to google but couldn’t come up with anything helpful.

Thanks!

A lot of people like to float mount including myself, I prefer it as I've seen to often prints framed with matts over their borders damaging the prints, leaving light burn marks, upon inspection usually when you are trying to sell an expensive print will have a light border around it when the print is removed. I know you'll get plenty of people to say "who carea abut value enjoy the art on the wall" Thats all well and good until you come to sell you expensive print, post photos of the print with the burn marks and then get comments complaining about how your print is damaged. If you intend to keep the print forever, then it doesn't really matter.

The only prints I get float mounts are as Romulus says, with deckled edges in sight. My view is that the artist chose to create the print with such an edge so it should be shown the way. If you go with a good Framer such as Mister Frame Man or PFG you'll be golden.

I've had a thought recently that artists are choosing to use deckled edges not so much for the aesthetics, but to be practical as there might be less chances in damage and replacements needed from shipping. If it's deckled, then perhaps the customer has less reason to complain about a little ding in a corner. Do you all think there's any truth to that?

Float mounting even when done perfectly will always mean a form of adhesive is touching the back of the artwork. It can be performed perfectly, almost without trace, but there's always an inherent risk.

There are other methods where this is not required.

Window mounts are fine with conservation materials, especially museum cotton rag board (no yellowing from acid migration), >99% UV glazing (ideally Museum or Optium) and careful placement out of direct sunlight, with mylar strips or corners. Done.

Get any float mounting done by a qualified ICON certified conservator, in my opinion, if the piece is of any value. I think most people get a framed piece back and assume everything is great, but fewer have revisited the work 20 years later and tried to reverse the process.

Some people think that window mount is a bit dated/old fashioned. Plus prints will inevitably fade slightly over time, so floating avoids any burn/discoloration or uneven fading with the Mount.

Float mount allows for the whole sheet to be seen - effective when the image is printed to the edge of the sheet or the piece is large.  
A couple of things to remember if float mounting - USE SPACERS! sorry for shouting.  Spacers go between the mounting board (which the art is floated on) and the plexi/glass.  Spacers as the name suggest keep the piece of art away from the glass.  This is important for the long term health of your art.  Paper expands and contracts especially when humidity fluctuates.  
One reason I like to use matting - aside from find it can enhance a piece - is that it allows for non-invasive mounting methods such as photo corners or "See Through Archival Mounting Strips".  One thing to remember is to allow a bit of space about 1/8 inch left, right and top for the paper to breathe.  Several posts on this section ask about rippling.  IMHO I would guess that the pieces are framed and have been adhered down on all sides.  This does not allow the paper to breathe and thus ripples.
Hope some of this helps.  I have more than 25 years framing and art dealing experience.
PS if you can always use acrylic/plexi.  It does not shatter and thus gives a greater degree of protection to your art.

full bleed prints are another reason for floating (Kaws)

A few years back I made a thread on one of my preferred methods of floating a print. You can check it out on the EB site.....the link is below.

forum.Expressobeans/viewtopic.php?f=31&t=62429&hilit=floating+a+print

And one of the benefits of using wheat paste and Japanese mulberry paper is that it is completely reversible. Seeing is believing. Have a look at this other video I made showing the reverse process.


And just to explain what I did in the video....I dipped the Q-tip into distilled water. A light touch of the distilled water is all it takes to break the bond. And there is absolutely no evidence that the paper was ever hinged. I've done this with pieces that were hinged decades earlier with wheat paste and the process is just as quick and easy. This is why using wheat paste and mulberry paper is the preferred method at museums (I work at a museum by the way). At the end of the video I'm dabbing the paper with a piece of blotter paper to absorb any moisture that might be left on the paper.

I have quite a few prints framed to the edge with spacers and haven't had any problems. The only issue I could see is if you squeeze the print to tight against the spacers I suppose there's a possibility they could leave indentations on the print border.