I implemented a firmware signing infrastructure for a large embedded hardware manufacturer using USB Nitrokey HSM devices (sub-$100/ea). We got the functionality we wanted from them at a fraction of the price point of the "big name" HSM manufacturers' offerings. The development effort to integrate the Nitrokey HSM, as a simple PKCS#11 device, was very simple. What we could get of the big-name HSM manufacturers' development docs, w/o an NDA or a purchase, looked to be much more difficult to integrate.