What you want is to add a calculated value control and enter a formula that calculates the maximum value in the Bid column in your sharepoint list, filtered to include only Bids for the current Item (I'm assuming that you also have an Item column in your list that is used to distinguish between different items). That formula will look something like this:
max(SharePointList/BidColumn[SharePointList/ItemColumn=CurrentItem])
Where CurrentItem is the value of the item the user is looking at. Use a calculated value control because it isn't user editable, so it will display the max but the user can't type over the max value.