The data rates are an absolute minefield; every network is different and they carefully avoid revealing how much more roaming rates are.
For example Vodafone.co.uk (click on Mobile Internet) offers £1/day unlimited data; this is NON roaming but they don't say so.
Similarly their £7.50/month contract with unlimited ("fair usage") data is NON roaming, but again they don't draw attention to that.
After a lot of work I found a page showing some roaming charges. These vary according to country and which network you roam with. For Austria:
£5 per day for up to 15MB
or £4.99 per MB
and that is just Vodafone. There are other networks, with different charges.
I normally use Virgin for data when I can because they are £5/MB for both roaming and non-roaming but it looks like Voda dropped from £10 to £5/MB recently.
Something worth considering is a contract, rather than PAYG. Not so great for flying usage because this is heavily concentrated into the summer, but if one can get a contract SIM card for one's normal phone, and then use that phone as a GPRS/3G modem, one is at least using the same SIM card for both jobs. Some of the recent contracts have lower cost data than they used to be for a long time.