Asafoetida is an edible resin - dried plant sap - derived mainly from the stem and roots of a plant from the Umbelliferae family (which includes other aromatic / edible plants such as fennel, dill and parsley, as well as a number of deadly-poisonous members). Asafoetida has a famously bad odour when raw but this changes on cooking - it's usually fried and used to flavour oil, or incorporated into a marinade - and has long been used as a spice in Indian and eastern foods.
When you buy it to use in cooking it comes as a slightly granular, beige-coloured powder, with a few noticeably larger grains in the mix. Most recipes that call for the use of asafoetida still include it only as an optional ingredient, and the powder is never used in quantities greater than 'a pinch of asafoetida'. I have used asafoetida on and off for years, and you need so very little of it that even when I was using it very regularly, I never got to the end of any given packet of the spice (I always found it would go out of date before I'd used it all up). It has a very distinctive (slightly rancid) smell when raw. In all honesty, I don't think I would necessarily recognize the taste of asafoetida in a cooked dish, but I understand it is said to add something to Indian recipes, so I generally include it if possible.
About12 years ago, when I was using asafoetida very regularly, I used to buy the 'Vandevi' brand powder (also known as 'hing,' to give it its Indian name) which I would get from one of the many Indian grocery shops in the Easton / Eastville area of Bristol. As far as I knew at the time, because it was the only company I'd ever seen selling the stuff, and because I'd never seen anything else sold by Vandevi foods, I assumed they'd pretty much cornered the supply market for asafoetida in this country. Their asafoetida used to come in a very distinctive, squat little yellow plastic drum, and 12 years ago, sold at about 50p for 50g. Vandevi asaoetida still sells in exactly the same format, for 55p for 50g. You can get it online for this price from such websites as eg. spicesofindia.co.uk (which I've never used, so I'm not in any way endorsing it....though I suppose it'll work out substantially more expensive once you've added on P&P).
I wanted some asafoetida the other day as it was one of the very few spices included in a mild fish curry I was making, and there was so little in the way of other flavouring in recipe that I thought asafoetida would be essential to the finished dish. At Tesco you can only buy Schwartz brand asafoetida (to be honest, it's such an obscure and little-used seasoning that I was pleasantly surprised to find that they sold it at all) and it currently costs £1.81 for 52g. I thought this was a bit over-priced - especially when I found that although the Schwartz asafoetida powder looks exactly the same as the other brand stuff I used to buy, it's not pure resin: it also contains ingredients such as rice flour, as well as turmeric for colour. Asafoetida resin itself, in fact, was one of the minor constituents of the Schwartz asafoetida powder.
To be fair, a quick bit of reading on the internet and I find that the Vandevi asafoetida powder I used to buy also contains a similar list of added ingredients - and just as small a proportion (presumably) of asafoetida, which may explain why the Schwartz brand powder looks so much like it. Although the two powders are much the same in appearance as well as cooking qualities, I can't see much justification for Schwartz pricing their product at approximately three times as much as the other one. Effectively what you're paying for in the supermarket is the Schwartz brand and fancy glass bottle. Although that said, the holey sprinkler-head in the top of the Schwartz glass jar does do a fairly good job of delivering just the 'pinch' of asafoetida that's all you'll ever need of the stuff at any given time.