The 30-second ringtones are available for $1.29 each. That's exactly the same price that you would pay to download the entire song track. The maximum length of a ringtone on iOS is 30 seconds.
The full list of 27 songs that are available as ringtones are:
"All You Need Is Love"
"Can't Buy Me Love"
"Eight Days a Week"
"From Me to You"
"I Feel Fine"
"I Want To Hold Your Hand"
"Let It Be"
"Love Me Do"
"She Loves You"
"The Ballad of John and Yoko"
"The Long and Winding Road"
"Ticket to Ride"
"We Can Work It Out"
Notably, if you've already purchased one of these songs, either from iTunes or from some other retailer, or ripped it from a CD you bought, you don't really need to buy a separate ringtone.
In short, on your desktop take the file, strip out 30 seconds of it or less that you want to use as a ringtone, rename the extension .m4r, and double-click on it in iTunes so it will be recognized as a ringtone.
Then, sync your iTunes library with your iPhone and make sure that ringtones sync. It'll be on your iPhone now and you can select it as a ringtone in the appropriate locations.
There are, of course, free apps in the App Store that will take a music file on your iPhone and perform the same operation on-device, as well (such as this one).
Frankly, no one should be paying for their ringtones if they have any technical expertise --- and not that much -- at all.