As someone who makes onesies and such I can tell you that if you are paying less than $25 you will be sacrificing quality at an exponential rate as you get lower.
Some companies like tykables, rearz or Abu for instance can make them a bit cheaper as they have the cash to have hundreds made at a time in a mass production factory since they have the rest of their product line to keep cash flowing.
The fabric cost for a plain solid colour onesie in a medium size can be anywhere from about $8 - $14 depending on type, quality of construction, colour, and if you buy in bulk (like 50+ meters at a time) add another 2-3 for elastic/binding/ribbing for the hems $1 for snaps $0.50 for thread and your looking at easily $11 - $20 purely on materials.
If it takes even just 1 hour to cut and sew and you want to make $10 an hour there's you $25 to $30 price tag right away.
Now if you want patterned fabric or embroidery/logos that cost goes up, especially if it is custom printed designs not off the roll prints.
Now consider that if as a seller you only make onesies or similar clothing your customers are not going to be coming back every month for more, especially if you use high quality materials. So you need to make enough extra to cover the time between sales as you won't have enough regular business to keep it running full time to cover the cost of living.