going to be setting up a killifish tank soon and have been looking on the net for breeders (only ever seen 2 being sold in shops) and a lot of people are selling 30 or 40 eggs along with food for the fry and instructions, is it worth paying less and getting the eggs? anyone had experince with hatching/breeding?

I buy killies all the time, from eggs by mail. It is easy, and not as easy as it looks.
First off, forget those deals. They aren't good. Use Aquabid and target species you want. The cheap deals are often mixed species. You will never be able to differentiate the females.
Are you going for annuals or plant spawners?
I keep Aphyosemions. Always buy lots of 15 eggs or more. From a great seller, one in three eggs hatches if you are lucky, one in five with middling luck and zero if things go wrong. A lot can happen in the mail. I don't sell eggs because the mortality rate makes you look like a crook. I've mailed eggs to myself as an experiment and had the eggs all consumed by bacteria before they got back to me.
That said, 2 in 3 species I buy get me breeding groups of fish I would never see otherwise.
Order from as close to home as possible, before it is too hot.

I have bought killie eggs off of ebay and Aquabid with excellent results. Good breeders know the length of time the eggs must incubate and ship accordingly. The problem I found with Asian breeders is that they will not or cannot respond to questions or provide instructions. Buying the eggs means you must be prepared to feed the hatch-lings immediately and consistently. I use Banana worms, micro worms and baby brine shrimp then move to daphnia, grindal worms and white worms so you must willing to give up your free time for a while to raise the food.
I have heard of nightmares from buying eggs off these sources but not found them to be true in my own experience.