Don, one high power pot will likely cost more than a single used variac and you will need one for each transformer. Then you have the heat factor. Variacs are very efficient and waste very little power. True a new variac would be very expensive but there are hundreds of used one out there.
How close are you to Van Nuys? There is an eBay seller there that sells refurbished items and lists many variacs. I have purchased a few lab power supplies from them. Very reliable.
Pete
