Depending on the breed, quantity and quality, hatching eggs are more per egg than hatchery day olds. When you consider shipping costs, you typically will pay $2 and up per egg and still may not get any live chicks. Plus you have the costs of building or buying an incubator and the electricity to run it, and your time and energy to turn the eggs, etc. I have an order now for 12 eggs for $25 total, about $2.25 per egg, which is a bargain for eggs. BUT I may not get any live chicks in the end, depending on fertility and my incubating skills, and the roll of the dice.
With hatchery day olds, you can get sexedchicks for less than $3 with shipping. The shipping is more expensive with day olds than eggs, especially if your order is less than 25 chicks, but if you order 25 or more, the shipping works out to be less than $1 per chick. For example, I ordered 7 day olds and the cost of the chicks was less than $20 total, but the shipping was more than the cost of the chicks. The total order was about $50 and I ended up getting 8 chicks total, my 7 plus a spare (in the gender and breed I had ordered rather than 'males for warmth' - thank you Meyer hatchery) - about $6 per bird, BUT I got healthy, live chicks. If I had ordered 25, my total cost would have been around $75 - only $3 per bird, and again, they would have arrived alive and healthy or I got my money back for any losses.
PLUS you can get them sexed, so you do not have to deal with the hassle and unpleasantness of culling or rehoming roosters if you can't or don't want any roos. Most hatcheries have a guaranteed 90% sexing rate and will refund money if they don't get it right.
If you are looking at cost alone, then hatchery day olds are the way to go. You get a sure thing - live, sexed chicks or your money back - and you get them quickly.
Eggs would be more for you if you want show quality, specific breeds that you can't get at a hatchery, or you are looking for the experience of hatching your own eggs.... OR you have a friend who will give you the fertile eggs for free.