Contact Sprint ask the about getting an Airave for free since your coverage is poor, I know they have comped a few ppl on BAW a free unit, so if they snow you and try to charge you for the unit or monthly service ask to speak to a manager or retentions, ps don't take no be assertive and firm.
An Airave won't do much for poor coverage for a mobile Broadband card.
There are several companies that sell in house repeaters ($400+) - they put an antenna on the roof, and a transmitter in your house.
There are also several companies that sell high gain antenns that connect to your card. They start out significantly less. One I have read about, but have no experience with and provide no recommendation plus or minus, is Wilson Cellular.
go to 3gstore.com. i have sierra wireless compass 597. had same thing going on would roam and get letters from sprint. i got a yagi antenna was $79.99 and with a cradlepoint ctr350 router $124.00 they are cheaper now. i have 2 laptops and a home pc running at the same time works great. i can stand on my roof and see the tower about 10 to 15 miles a way. a majer change very easy to set up.
The best solutions I've seen for this is cst's that I've talked to that have an outdoor antenna. Now it won't bring in a signal if you're not getting one outside. The main point is to remove as many obstructions as possible and get the receiver away from interference sources. I've seen some situations where the wiring in the walls and electronic equipment was causing so much interference, that the signal increased 20 dBm by just getting it outside.