There are similar threads on this type of topic. I suggest looking up the RIAI fee surveys to get a feel for whether you think you are being over charged - it is impossible to say that 20k is outright too much for planning as we do not know the nature or extent of what's being planned, we also don't know what type of architect this is. To use an analogy - you can't say that 20k for a car is a rip off without knowing what type of car it is - it's brilliant value for a brand new BMW or Aston Martin, but poor value for a 20yr old mini etc.
Secondly - it does not make sense for someone to assume the risk of a 3rd party - in this case the planning authority. It is NOT the architect's responsibility to achieve a grant of permission, it is only to submit a design that the client is happy with in a format that may be determined a VALID planning application. It is the industry norm to charge for substantial Further Information requests.
I suggest examining the Terms of Engagement to which you signed up under your Client - Architect Agreement. The RIAI standard terms of engagement and agreement may be found on-line.
Architects, like most businesses, don't charge a bucket of money for something and then try and get away with it - there is open competition in the market place and generally services are priced in and around their cost to carry out. So if the application cost 20k, then odds are in wages, insurances, capital, office maintenance pensions etc etc - that that application cost the architect 20k to produce. The typical profit margins in construction are between 3 and 5% - so, if a whack of Further information comes in - unless you expect your consultants to do it at their own cost...they need to be paid to do it.
And finally - in terms of cost/time, architect's are the cheapest of the construction professionals.