The problem with getting a generic SIP phone is that although the system supports them (depending on software level), they need to be licensed individually and if you don't have any licenses already, this is usually not a good way to go since it will require a technicians involvement (and thus an service fee charge).
But if you are looking for a cheap phone to place in a house, search eBay/Amazon or an online refurb house for a Mitel 5304, 5312, or 5320 and you can probably come up with something for under $100 and then all you need is a Category D license (the most common) to make the phone work.