Baby soft-touch coveralls and rompers on sale for $5 at Target
Having a quick look online I can see that there are 9 items for $5 and a further 23 for under $6.30 (boys and girls). So plenty of cheap options to choose form.
If you order online and want them delivered, the shipping fee is $9. So it's best to either buy a larger number otherwise you're you could always go for the click-and-collect option to keep the prices low.