So far the main plugin I've spent money on is Advanced Custom Fields, purchasing the repeater field and options page add-ons. Cost around $40 USD I believe. Gives me a license to use those add-ons on as many client sites as I want.
I've also had clients purchase WooCommerce extensions like the PayPal Pro gateway ($79) and Stripe extension ($79).
Also, as I continue to build out http://BostonStartupsGuide.com, I expect I'll purchase SearchWP ($25) and Events Calendar Pro ($65).
If it's some small bit of functionality I will try to write it myself or do some creative copy/pasting. Most plugins are bloated with a lot of extra unnecessary stuff.
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http://wordpress.org/plugins/kennethrapp
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