Well... if I'm not completely mistaken on your setup, can't you buy HSDPA cell phone, place the SIM into the phone, and pair the phone with N810 using Bluetooth? (This is assuming your HSDPA modem has SIM card.)

Then you can use the phone as your HSDPA modem wirelessly.

If your budget is up to $1000 you should have plenty for BOTH N810 and 3G (unlocked) phone.