Don't forget he has officially been our player since his loan ended before the play-off's, and will be till July 1st. So would need to include May & June in those figures. I doubt Palace are still paying his wages.
15k * 32 weeks (ish)... = £500,000ish.
Remember just because they are free agents it doesn't mean they are 'free', I would expect the likes of Bridge would have got a hefty signing-on fee from Reading for example, and wages are usually higher on free agents too.