A full FMS, new, is an expensive proposition. However, depending on how far you want to go and how much you want to include, there are very inexpensive nav and communications options that you can use.
I used a Garmin 430 and 530 in a Lear 35 and it did very well. I'd prefer a UNS for that application, but the Garmins did fine.
GNS units are less in favor these days; you may be able to pick some up for less change than more up to date systems. Then again, depending on how much you want to spend, you can outfit the entire panel as glass, and include a FMS with the works for a price...and depending on how close you want to come to that, the price goes down from there.
The best bet is to identify the specifics of what you want, and sit down with an avionics shop to explore the options and their recommendations. As pilots, we'd love to have all sorts of gadgets in the airplane, but it's not always cost effective, and the installation isn't always practical. With the wide array of avionics options available today, some of the slightly older systems are being replaced, and can be had for an excellent value. A collins 85 EFIS suite can be a bargain if you get it as a used unit in something that's upgraded. Check with your avionics shop to see what they have available and what they recommend.