It depends on what you're doing and how much of the site it is. An add-in might be an almost totally separate product already, just one that's leveraging the ex

It depends on what you're doing and how much of the site it is. An add-in might be an almost totally separate product already, just one that's leveraging the existing infrastructure of WP such as logins, permissions, theming, etc. If it's just leveraging those things (and thereby changing appropriately if you change appearances) then how important is it if it's written in PHP? I will say that if you're spending 10k+ on add-ins that you should probably make sure you've looked into what you're doing and how. That sounds like the kind of pricing I'd expect if you're having a custom add-in developed rather than what I'd expect for a commercially available one. Something custom may be appropriate, but unless your needs are unique it may not be the best approach.