How Many Guineas in a Gross of Threepennies?
Ruby, 345 306 302 288 287 278 273 253 252 242 232 221 202 190 bytes

f=-&gt;s{" !#+/7OďǿȗϟЏ'"[%w{fa fp ^pe|co r..p ^gr x|ta sh|^b fl|b f.c ^c f.s .gu d|v ^g .}.index{|k|s[/#{k}/]}].ord-31}
$&gt;&lt;&lt;gets.sub(/ (.+ i)n /){" #{r=$1}s %0.4f ".%$`.to_f/f[$']*f[r]}


Takes input from STDIN and prints to STDOUT.

I'm using short regular expressions to match only the desired denominations for each value. There are two arrays, one with regexes and one with values, at corresponding indices. The regex array is a space delimited array literal, and the value array is packed into a string of UTF-8 characters.

I'm selecting the index into the values by searching for a regex that matches each denomination. I'm also defaulting to the tuppence/half-groat case (value 8), because that required the longest regex. Likewise, some of the patterns assume that other values have already been matched by earlier patterns, so each regex only distinguishes the desired value only from the remaining ones. Using this, I could probably shave off another couple of bytes by rearranging the order of the denominations.

Thanks to Ventero for helping me beat Pyth making it shorter!