If the cross is temporary, then a nice knot-free piece of pressure treated pine, 100 x 50 mm should do the trick. That's lasting enough and looks okay, especially if you add a staining, weatherproof finish to make it look like cedar or some-such. As for your labour, I wouldn't presume to tell you what to charge, but I should imagine it wouldn't take you long to make with a router.
PS I just thought... Why not go to your local cemetery and ask there if they know of anyone who makes wooden crosses. Perhaps they could tell you what the going rate might be. Just an idea.
Your hourly rate x hours it takes, as its a mate round down and knock off a few quid. If its timber you already have charge half the value to whats used and if not charge what you paid for the timber. Thats how I would do it.