msmcleod
Synthmaster would be my first suggestion, but also checkout AIR Xpand! 2, which is quite often on sale... like it is now for under £4: https://www.pluginboutique.com/product/1-Instruments/64-Virtual-Instrument/1560-Xpand-2
I have SynthMaster, Hybrid 3, and Xpand!2. I wouldn't actually classify Xpand!2 as a synth, as it is more of a ROMpler, although stuffed with a wide variety of sounds. Very useful for quickly sketching up a few tracks! But for someone that already has Kontakt, it is probably mostly an overlap, regardless of the price point.
If you want to keep the budget under $1, you cannot go wrong with Hybrid 3. A lot of modern synth sounds to be had with that for the price!!! It really has a good sound. with two layers and two step sequencers. Great for moving sounds, sequences, arpeggios, etc.
But SynthMaster overall for the value that you get is hard to beat! There are multiple synthesis types available, including classic virtual analog and wavetable.
SynthMaster also a lot of very good preset expansions from skilled sound designers as add-on options to go with. They cover everything from the vintage to the modern sounds, and the expansions are available individually or as an "everything" bundle.
http://www.synthmaster.com/