Thanks, CarolMN! That's THE best-and-cheapest way if you don't have tickets yet!
If you do have tickets, the cheapest way is by public bus. You'd need to get to the TTC, so WDW bus to Magic Kingdom then monorail to the TTC. The bus system is called Lynx, and you'd start with the Route 50 bus. You'd need to transfer somewhere, and do some walking. The fare is $2 per person, and I don't know if they allow free transfers. If they do, it'll cost you $2 per person each way, or $24 total; if not, double that. See
www.golynx.com for exact routing, transfer, and schedule information. ETA: This price may sound appealing in comparison to what follows, but it's a lonnnnnnnng trip.
Alternately, you could go with a car service. Tiffany Town Car (for example) is $90 round trip for the van you'd need. A taxi would be about comparable, but because the fare is affected by things other than simple mileage, it's less predicatble. From a mile and a half off I4 on 192 to Universal was $38 and change a couple of years ago.
There's also the Mears Shuttle for about $20 per person round trip. As you can see, that's more expensive than the above options, but - and I admit I get my transportation options confused
- there MAY be no charge for passengers under a certain age.