PluginBoutique currently has a £1/1USD deal running on the Hybrid 3 soft synth. It's actually listed at £0.75 at the moment, so even less! Not sure how long it lasts for but it says it's a Black Friday deal, so I've set it to expire on Saturday - your mileage may vary.
It supports VST, AU and AAX plugin formats. You'll need a music production suite / DAW like ProTools, Logic Pro or Ableton to be able to use this. Note it also uses iLok as copy protection, so you'll need an iLok dongle or the software "dongle" (management software) installed on your PC or Mac. Pretty common for this sort of plugin.
I haven't personally used it but I'm planning to grab it and try it out. It had a positive reviews here and here.
If you like it there is also a £4.50 (A$7.87, 98% off) bundle including 5 extra plugins, which can also be purchased individually at a big discount if you prefer:
- SoundSpot Propane - mid/side panner / stereo placement mixer
- SoundSpot Overtone - EQ with a deliberate analogue feel
- SoundSpot MSW1 - stereo placement mixer with automation support
- SoundSpot Percolate - synth and acoustic drum mixer, snare, hihat and percussion
- SoundSpot KickBox - similar to Percolate but for kick drum mixing
I'm not saying these are the best at what they do or there aren't other alternatives but they seem well reviewed (even at full RRP) and it's only a few bucks instead of a few hundred!