I know of these 3 options for the BMS for BMI batteries.
BMI VMS
The manufacture of these batteries provides what they call a VMS. It's actually a microprocessor based board that handles balancing and gives outputs for driving external circuitry for over voltage and low voltage protection. The board they provide is able to monitor 4 cells in series, you can connect multiple boards via a provided port so that you can build up a system to monitor as many batteries as is needed. The single VMS board is around $50, so for a 36V system you'd need 3 boards so about $150. This setup is nice since it monitors the batteries constantly and logs over volt, low volt, cycle count and other events into internal eprom. There is also a serial interface port that allows you to connect you computer and read the stored data. It's biggest limitation is the balancing current which is about 100ma, but since these batteries seem to stay pretty well balanced, it's not usually an issue. Also, using this option, you'll need either a smart charger or an external circuit to cutoff power due to high voltage and you'll also want an external circuit for cutting out power due to low voltage. EchoCar has a great write up on this VMS board. Electric Echo VMS link
PING BMS
Li Ping provides a good solution with a complete board that can be wired directly into the pack. Their board also comes complete with High Voltage Cutoff so it prevents over charging by cutting the charge off to allow the circuit to balance the pack. It also has a low voltage cutoff circuit to disconnect the load when the battery voltage is too low. This is the lowest cost solution for a full bms. Ping sells the complete board (for a 48V pack) for around $50). This solution is about 1/4 the price of the BMI VMS solution. However, this is an analog circuit that does not do any logging of data conditions. Also, since ALL the load current must go through the LVC of this board, you'll need to be sure to request a board that can handle the maximum current you'll ever need to use. Since the BMI cells can source 200Amp or more and the ping pack is currently limited to about 50Amps, this solution will not allow you to use the batteries to their maximum potential. A possible solution would be to wire 2-4 of these boards up in parallel which should increase the total load current capacity AND the balancing current. I haven't tried this yet, but it should work fine. Li Ping is a very reputable distributor and I've had good dealings with him. He can be reached by email. pingping227<AT>hotmail<DOT>com
4-24 - Cell Battery Management System
OK saved the best for last. Here is a kit that was designed by an endless-sphere forum member to be the best bms available. It has a completely flexible design that can be used on packs of any size. It's for the do-it yourself-er. You'll need to solder it up, but this saves in cost and allows you to create the circuit that you need for your particular application. You'll also need to get your own components which can save you money if you a lot of parts laying around.
http://www.tppacks.com/products.asp?cat=26