I suspect the cost largely depends on the existing network provision. Are the HoC and parliamentary offices covered by an existing secure wifi network? If so, there's no definite requirement to issue 3G devices at all, they can supply wifi only ones. In fact as a security measure I'd be inclined towards wifi only iPads with mandatory VPN configurations running. If they want to use them elsewhere, they can find a hotspot or use tethering from a phone.

You can provide a reasonable facility while at the same time minimising costs, in fact that's part of a sysop's job. And I don't think there are many MPs daft enough to pop their head above the parapet by complaining about such an arrangement, to be honest.