mmm hard one, depends on what the 1in holmes is, if its a mk 1 or mk2 then its worth more than the 1/18th if its a mk3 1in then i think they are worth about the same maybe a little bit but not much more than a 1/18th as the only difference between the 2 are the headtube sizes iirc
as for prices id say about £150 maybe a bit more depending on colour