Which model sensors? The Simon sensors I’m aware of are pretty much in the same size range of typical zwave and zigbee sensors:
http://www.interlogix.com/intrusion/category/wireless-sensing
The one exception is their Micro Sensor line, but the NYCE sensors that work with SmartThings are about the same size, maybe even a little smaller.
And the fibaro motion sensor, at one and three-quarter inches across, is smaller than anything that’s a motion sensor in the Simon line that I’m aware of. Granted, that one’s expensive.
The Lowe’s iris zigbee motion sensor is almost as small and can often be found on sale for about $20. Again I don’t think there’s a motion sensor that small in the Simon line. The Lowe’s is on the left in the following picture, and the SmartThings motion sensor is in the middle. The right is the same size as a typical Simon sensor.
http://www.lowes.com/pd/Iris-Motion-Sensor-Works-with-Iris/999925310
Wireless tags from Kumostat also work with SmartThings through some community written code and are very small, very nice devices which work well as door sensors. But again on the expensive side.
Bulk Buying?
As far as bulk pricing, that’s a little more complicated. The Simon system, of course, is typically sold by companies that require a monthly service contract, so they can afford to subsidize the individual devices somewhat.
With Smartthings, you’re not usually paying a monthly fee and you’re buying most of the devices from third-party vendors, so you have to carry full weight for the price. It’s like the difference between getting a subsidize cell phone from Verizon and buying one at Retail.
So you’re right that many security system vendors sell the devices more cheaply, but it’s not because they’re proprietary – – it’s because they’re making it up in the monthly subscription fee, just like the cell phone companies.
You may be able to get a volume discount at Lowe’s on the iris devices if you check through their contractor desk.
But you can still find some volume discounts if you shop around.
www.zwavedevices.com Will also often given volume discount, you can check with them if they have a particular model that you’re interested in.
Kumostat has a 20 pack Price on some of their sensors that typically runs around $399, although it depends on the model. But you also need to buy their bridge which costs about $60. So their bulk prices are around $23-25 per sensor depending on the exact package. You can typically get the iris devices for less on sale.
Other than that, you just have to look at the total annual cost and shop around for bargains. Several of the online specialty retailers will give you a bulk price if you’re buying at least a dozen of one model, but you usually have to ask about it.
If you can be more specific about the use case you’re looking for, including the budget, we can probably help more.
For example, one of the recessed door sensors won’t show at all since it fits inside the wall, and you can sometimes get a bulk price on those.
One of the strengths of the SmartThings system is the number of different devices it can work with, so if you have a specific use case in mind we can probably help recommend some possible devices. 