Hi wjvii,
This depends on your environment. If you only need a small number of ports (up to 8 channels) and you have a ISDN PSTN -> Dialogic is offering a very interesting product (DIVA 4BRI ISDN Adapter 4 ports / 8 channels). In the future it will be shipped with SIPControl included so you don't need to buy additional licences for SIP2PSTN gateway. I don't know pricing for this product exactly but it should be lower than $1800.
This product will also be available with 120 channels and additional licenses for SIPControl.
Both can be used out-of-the-box with Exchange 2007 or OCS 2007 using one or more Mediation servers for redundancy.
HTH, Captain MESO