What public transport options are there between Universal Studios and Disney World? How much would catching a cab cost?
Mears runs a shuttle between Disney and Universal - but if there are more than three in your group it is almost always less expensive to take a metered taxi. (Up to seven can ride for the price of one, depending on the vehicle). You can ask for an estimate of the fare before accepting the taxi.
Looking around online, TaxiFareFinder claims it'll be about $30 one-way between the two locations.
You can also bus between the two for $2.00, but note that it'll take considerably longer (1.25 hours vs 17-20 min), and you probably want to maximise your time at the parks :)
I'd say that Google Maps is your friend with respect to public transportation. According to Google the cost of the trip is $2.00