I think the ultimate suit knife is a CRK Mnandi. The pocket clip is almost mistakable for the clip of a pen. But...that's way above your price range.
For thin, lightweight dress knives I have several in addition to the Mnandi. The following are mostly around your price range...
1. Spyderco Chaparral. I prefer the LWT version but the CF or Raffir Noble versions might be a little "dressier".
2. Isham Bladeworks Blackstar. Probably the closest you can come to a "poor man's Mnandi".
3. Benchmade 707 Sequel. Hard to find sometimes. I'm not sure if this model is discontinued or not. Axis lock goodness in about as thin a package as you can find without the awkward shaped handle of the 530/531 series.
4. ZT 0450. This knife is a flipper only. I usually like a suit knife that I can open quietly without too much flashing steel and lockbar clunking into place. Get the CF version or add a CF aftermarket scale for a more upscale look.
5. Benchmade Mini Griptillian. For this I have a specific custom Grip. It's a 555 model with the hole opener, mounted in some custom, dimpled aluminum scales with all blue hardware. It sits much slimmer than a standard Grip scales and it makes a great suit knife.
6. Any kind of GEC traditional with a leather slip sheath. You'll want to stay fairly small and lightweight so it doesn't hang like you've got a banana in your pocket.
7. J.E. Made Zulu. This is available with or without pocket clips and sports a traditional lock with modern materials.
8. Hiroaki Ohta OFF FK7 in your choice of handle material. A modern take on the classic Higonokami design. Carry it in a lightweight leather sleeve.
9. Berg Blades Iron Pup. This is about as heavy and clunky as I get with a suit knife but it's such a well made little knife.
10. Three Rivers Manufacturing Neutron. This is not last because it's least. I put this last because TRM knives are just so hard to get right now. The Neutron is perfect size and weight for a suit knife and they come in a variety of handle materials to suit your personal taste.
A couple of generic considerations to consider...
Weight is critical. I try to stay at or below 3 ounces. With winter weight suits you can probably push the weight up but with summer suits the weight will really affect the hang of your trousers or jacket.
Size is also very important. You don't want to print that you're carrying an Espada XL. I really try to stay below three inches blade length and I'm actually fine with just a bit over 2". The thickness/height of the handle is as important as the length.
Try to pick a more discreet, civilized look. Avoid black knives and try for lighter colors with satin finished blades. You don't want your tactical, zip-tied knife of doom to become the topic of discussion at a wedding reception when you "whip it out" to open a package. This is where traditional knives are often a great choice. Nothing screams wholesome America like an old-fashioned "jack knife" with stag handles.
Try to find a knife with a silent opening mechanism. Thumbstuds, holes, disc, nicks all allow the knife to be opened slowly and quietly.
Be mindful of the pocket clip. Often, that's all that someone will see of your suit knife. If they mistake it for a fountain pen, perfect! And you certainly don't want it decorated with grinning skulls ala DPx.
I have to carry a "suit knife" every work day, so I tend to compensate and go BIG when the weekend rolls around
.
