Used S&W 5906
I was checking the used pistol selection at the shop next to work. They have a used S&W 5906 in stock with 1 15 round mag for $350.00
I'm assuming that's about a fair price, correct me if I'm wrong.
Second, I'm looking at trying to do a trade for it with my Kel Tec PF9.
I'd say a fair guesstimation would be that they give me $150.00 for the Kel Tec leaving me $200.00 in cash to come up with.
As much as I like the 5906, I would not use one for concealed carry. They are a little heavy for CC. Open carry, sure - with a good solid retention holster like an old school Safariland.
I would recommend a 5903 (SS slide w/alloy frame) or even a 915. Both are quite a bit lighter and you get the same quality & functionality as the 5906. Also, there is currently a flood of S&W 915's in the market as a result of police-trades. Some of these guns are pristine internally with a few wear marks on the outside. There are even some that are literally new on the outside as well. The 915's can be had for about $250.
With regard to your KT PF9 - You can probably get more than $150 if you sold it yourself. Depending on condition, $200 would not be unreasonable. I picked mine up NIB for about $270, including tax.