Why is this? Are you on a wireless ISP plan? Or do you herald from a nation with poor access in general?
A first-world nation with third-world internet access due to a telco monopoly.
Do you mind telling us the nation in question? I might have a solution for you to pursue, depending on where in the world you are. And are you an urban dweller?
New Zealand, and yes.
NZ is good. Urban is probably good. You might want to consider setting up a mesh node filtered for only Bitcoin traffic, and annouce in the area that you are doing so. If users join the mesh, bitcoin traffic to the outside world can be shared (if you're careful about it) and most of the traffic that your personal node would see would be coming across the mesh. The trick is to force your local client to only permit one or two connections from the ISP and the rest from the mesh, and as long as others agree to do the same and don't freeload (not a given), your own personal chargeable traffic from Bitcoin should be managable. You should throttle the ISP Bitcoin traffic anyway, down to modem speeds; or set it up to only connect to ISP side nodes at particular times of the day. In the future if you have a portable client on a smartphone, set it up so that it only connects to the mesh clients via wifi and never over the cell network.
Better yet, try to convince your ISP to set up a node of it's own that you could connect to without charges. If traffic beyond your ISP is what is getting you charges, they might be willing to do this because such a local node can function well as a caching gateway, permitting the ISP to block outbound Bitcoin connections without actually interfering with the Bitcoin network. As the Bitcoin network grows, I have no doubt that ISP's are going to want to control bandwidth consumption.
Of course, I have no idea how receptive they might be to the idea, but it's worth an email.