Congratulations. Welcome to the jailbreak community.
You don't pay with Installous, you just go find the app you want, go to download and choose a link to download from... they'll be like fileshare or megaupload or something like that. By the way you can also download the .ipa files from http://apptrackr.org on your computer and add to iTunes or add via SSH to your Installous downloads folder. You need to have iTunes Sync turned on in the Installous settings btw to transfer them.
Well you are welcome to buy some of the sweet Cydia programs (.deb files), and there are some themes you can buy as well... the blue links are the paid ones btw. However...
If you go to the Manage tab, Sources, Edit, Add:
http://sinfuliphonerepo.com/
and
http://apt.xsellixe.com/
Those are two of my favorite sources, also called repositories. Sinful hosts cracked Cydia store apps, and Xsellize hosts cracked Cydia store apps as well as cracked themes. So just like people crack .ipa files and put them on Installous, people manage to crack .deb files. Some haven't been cracked yet like Shrink for iOS4.
K hope that cleared it up.