I think you shouldn't worry about your Host undercutting you, as a reseller you always have an edge over it such as sell hosting packages slighly lower than your Host, unlike your Host they usually have to take into consideration that they have reseller and pricing their Hosting packages very low defeats the their plan/sytem of having resellers that contribute to their earnings.
On the Host's side, they benefit either ways so it's not in their best interest to compete with their resellers in my opinion.
As an example, say you are a dealer for Sony products and Sony usually have standard pricing that they sell to consumers, going through the dealer will get you a better discount for the same products with the same warranty.
typically it works two ways for a reseller vs their provider in competing for clients:
1. A reseller will almost always get the smaller accounts, if you're a provider, you've got better things to do then to collect on $5/mo accounts.
2. A provider should, and typically will outcompete a reseller for larger accounts, ones that are actually worth the hassle, a reseller can always oversell and hope the client doesen't use it, undercutting the hosts price, but this is more unusual for most.