If you’re going to replace them, it’s definitely best to do everything at once; you’ll save on the crew’s call-out and delivery fees, plus you can negotiate a good discount for the larger volume of work. As for pricing, it really depends on how many non-standard openings you have: standard frames are reasonably priced, but you’ll have to pay extra for panoramic or corner windows. Before placing our order, we used a Window Replacement Cost Calculator to get an idea of the average price per window and for the whole house. We ended up splitting the payment, but we did the entire cottage in one go - so we saved money and didn't have to deal with the mess twice.