I can't tell you about cheaper flights, but if you check in together and ask nicely, you should be able to sit together. This is of course assuming that the airline you travel with doesn't assign seats when you book. I know the BA don't sort that out until check-in, but easyjet assign you a seat from the outset.
With this in mind, I would suggest that you check in as soon as the desk opens so you can be sure, or use BA's 'eticket' system as soon as allowed. I don't fully understand the BA e-system but I would imagine it gives plenty of flexibility.