Hi and welcome to GOY
in my experience, buying bulk is always cheaper. but you have to consider how much you need, if you can fetch it yourself [as in what is the delivery charge], and how far you need to travel. As for the volume needed, if you are familiar with the bulk builders bags, they are approx. 1cubic metre=1000 cubic litres/ which in the case of sand/gravel = around one metric tonne. I imagine soil supplied in bags will be less than this as soil holds more air [bulks up] when loading shovelling etc . With most things in bags, it sometimes depends how wet the materials are when loaded, as in loaded wet you will get a little less due to weight of moisture.
to get your volume take your length [L] multiply this by width [W] and then multiply this by depth [D] . This done in metre measurements will give you the amount of cubic metres. Not sure what the weight of a cubic metre of soil should weigh, I`m sure others on this site may be able to add something more specific.
Hope this has helped towards your answer
Cheers kev

Just make sure that what you are getting is good quality top soil, there are roughs out there that will try and sell subsoil as topsoil!

The weight of soil depends on how wet it is. Soil holds a lot of water so if the soil has been lying outdoors in the rain and you are paying purely on weight, you will be buying a lot of water! On the other hand, I would not buy the silly little bags from the garden centre.

For a small raised bed, 2 or 3 bags of topsoil should suffice. Here in US, each bag is 3 cubic feet. I usually top off with 3 bags, plus a bag or two of composted manure. You can add sand of peat to improve drainage.

Top soil I bought for my raised bed was awful, it was full of clay particles so buy from a good source, ask around, my Dad used to test it by putting some in his hand & would spit in it and would squeeze, if it stuck togeather he would send it back LOL.

Many thanks folks some great answers and advice. It's all appreciated.

Our local skip gives you free compost.

And when we were walking along the beach at St Andrews you could also collect free compost.

IF you are walking along the beach, get some of that seaweed that washes up on shore. It makes a super compost

I must check that out Scotkat. I live in North Lanarkshire and they collect garden refuse to compost every fortnight but I haven't heard them giving it out at the skip.

Bathgate - great idea. I will do so the next time I am there.

Kath's idea is great. Just be careful buying topsoil. When I moved here 2 years ago I soon discovered that the last owner had been fobbed off by his contractor with solid clay subsoil for his raised borders. Really ridiculous as we have some of the best topsoil is Scotland here in Angus! If it were me, I'd try the local quarry. We have a good one in Barry just down the road and their prices are great.