There really isn't a way to say that, nor is there a way to politely hint at "better" gifts. There's also nothing wrong with providing a less expensive and less personal gift to the unmarried partner / fiancee / signficant other of a family member. In fact, this is quite traditional -- until one is married and therefore part of the family, a more modest gift can be much more comfortable for all concerned. A girlfriend / boyfriend / significant other / fiancee may be made uncomfortable by extensive and expensive gift-giving, and on the family's side it might seem inappropriately intimate before a marriage. You may find LordL's parents are like mine, and while presents were more modest prior to my marriage, now my husband gets more thought-out and more expensive birthday and other presents than I do (including routine I-saw-this-and-thought-of-you gifts from my Mom, of which I get none.)
From my perspective, I would not give, for example, my brother and his girlfriend (who I very much like and want to become my sister in law) a couple gift as they are not married. I think many other people feel that way - it's not right or wrong, it just is, and unless one knows that the giver is comfortable with suggestions and couple gifts for unmarried couples, it can be an awkward thing to ask for.
Finally, I think a lot of understanding is important at Christmas - and to assume the best of intentions. A $15 present from my perspective is not a cheap one. I also recall a thread a few years back that got incredibly vehement regarding providing stockings for significant others sharing the holiday - those who thought that they were a nice inclusionary touch, and those who thought that they were presumptuous, rude, and insensitive to hang a stocking.