How can I get a list of all my paid (only) apps in the Play Store? I'm interested to know all of my apps that I paid for in the Google Play Store. Is there an easy way to retrieve this list filtering only paid applications?
(1) Check out Legacy Play Store. You can get the list on UNINSTALLED PAID apps. INSTALLED list includes both PAID and UNPAID.

If you decide to install it, you may need to: 


use Root Explorer (I had troubles with Astro and File Expert) 
copy the apk to /system/app 
chmod the apk to 644


NOTE: It works on my Samsung Galaxy S2, running CyanogenMod9.

EDIT: Hmm. Website is currently unavailable. Post a comment if you can't access it. I could probably upload the apk somewhere. [Or you could search for the app elsewhere]

(2) AndroidPolice just featured a GreaseMonkey script that allows you to separate your paid apps in Google Play. You might also want to check it out.