what should I replace "as" with?

[vkhu]

"The prices at the chain stores are as reasonable, if not more reasonable, as those at discount stores"

In my book the key state the bold as is the wrong word but I have no idea what would the correct one be and why is that word even incorrect in the first place. Help please

 

[5jj]

It should be: "The prices at the chain stores are as reasonable as, if not more reasonable than, those at discount stores"