Dongguan is one hr. drive from Pazhou Canton Fair venue, (it varies base on different part of Dongguan). For some part of Dongguan it is just 40 min. generally speaking, the room rate in Dongguan during Canton Fair would only slightly increase 30%, some of them would even keep the normal prices.
The room rate of a 5 star hotel in Dongguan would ranges from US$100-150, which is half of the price in Guangzhou.
Some of hotel in Dongguan will offer free shuttle buses to and from the venue of Canton fair exhibition. It is better to choose one hotel in Dongguan which offer this kind of service.
More detail you can check:
http://www.ecomeego.com/Accommodation/Dongguan%20.html