I've been researching the sample library market for a long time to try to find the best deals on the best products, and I've found some very affordable gems. Here's a small list of inexpensive but high quality libraries. Feel free to contribute in the comments.
If you want a rich, lush, and expressive-out-of-the-box string library, this is a great option for any budget. The entry level pricing is not indicative of its utility and quality. There is no legato, which is an important feature, but you can easily work around that. This library is ideal for soft cinematic and piano and strings pieces.
This is a magical, earthly sounding flute with convincing vibrato and legato. It sounds good out of the box. It was recorded dry and has a built in reverb. If you opt to use your own reverb, I recommend adding early reflections reverb prior to adding tail reverb - adding a tail to a dry sound isn't ideal.
Embertone's flagship piano is one of the very best in the market, and you can buy it at a significantly lower price than many of the other leading piano libraries. It has a neutral but very pleasing tone - it's not ideal if you're seeking a piano that will add distinct character to your piece. It's extremely playable, and I find it requires a lot less humanization than other piano libraries. You may also want to consider this piano library from spitfire.