It seems to me quite easy to specify source/ target pricing and other important factors (method of payment, how and when made, etc) in the body of the bid message. There's certainly enough space, and a lot of matters to be clarified in bids (representations regarding format of original text, delivery, option to decline if..., etc).
Not to mention, of course, the compelling reasons why you should be awarded the bid.
So what happens if the bidder fails to specify whether they are quoting on a source or target word basis? I realise that there are language pairs in which this does not make a great difference, but there are also pairs where a very different fee will be payable depending on whether the source or target word count is taken. Is it not possible that a bidder may take it for granted that their quote is based on, say, the target count and the client just assumes that it is based on the source count? This is surely creating the potential for disputes to arise which could easily be avoided if the bidder were required to specify which word count the bid was based on.
Another alternative would be to have a clearly visible notice at this place on the bidding form stating that one of the two options applies as a default, i.e. unless stated otherwise the bid will be based on the ..... word count.