I do a lot of Wordpress development for clients. I've spent more on themes (between $30 - $80 per theme, usually closer to $40) than I have on plugins, and the themes often include functionality I'm looking for that I might otherwise use free plugins to create in a theme I'd build myself.
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).