1) Data-only device modules are made by companies like Telit, Siemens, Wavecom, etc. To use one of these, you have to do hardware design as these modules are intended to be a "component" of a larger design. These cost between $60 and $100 depending on feature and volumes.
2) More complete units are made. These usually have a serial interface, and an antenna connector, and come provided with an antenna. Give it AT commands, and it sends data. Costs are between $200 and $500. Device in this group are made by Multi-Tech, Airdesk, etc, etc.
Cell phone costs are pushed-down by the cellular companies. Their business is based on selling the cellular services and options - even if they sell the hardware at a loss. Whereas, the data-only business of 1) and 2) above must make their profit on the hardware.
Another big issue on the hardware is the approvals on a cellular device. There is currently some lawsuits and congressional pressure on the cellular carriers and phone manufacturers. To 'control' the market, you have to meet their standards to have a phone on their network. There is some question as to whether they use this control as a method of restricting the market. Even a biggie like Apple with their IPhone had to run the gauntlet to get carrier approvals which delayed the introduction of the IPhone by several months. Cellular devices can be approved for an individual carrier by meeting their testing. GSM Cellular devices must meet PTCRB standards. CDMA device must pay Qualcomm several dollars royalty per phone to use IP.
Even if you use a pre-tested/certified module of 1), you must still repeat the certification testing for your application, costing from $20,000 to $45,000, or half this to get approval for a single carrier. Devices of 2) come with the certification, but if you so much as even use a different antenna, you are required to re-certify! Now, if you want to buy the cellular chips, and roll-your-own, expect costs of $200,000 to $500,000 for development and certification.
I've only talked about the hardware. Similar issues arise to arrange for a carrier to recognize your device, carry your data, and collect their money.
Welcome to the Cellular Data World!