Single wall, double wall, handles, personalised?

What you intend to do with them will determine the spec you need. e.g. are they for lightweight things that don't need much protection or heavy and/or fragile items that need enough protection to pass through courier systems relatively safely.

If the size you need is approximate look up people such as kite packaging or similar but for made to measure look for cardboard box manufacturers in your area (or nationally but I try to buy locally), the bespoke option may even work out cheaper, especially if you have to add the cost of voidfill by using oversized boxes.

Price will vary depending on spec but you'll probably get something lightweight at well under £1/unit up to £1.50/unit (max) for thick double walled very sturdy boxes.

Box manufacturers generally talk about internal dimentions rather than external and quote per 1000 (doesn't mean the minimum order is 1000). If you can be flexible on quantity let them know and quote you accordingly, most buy in board for the job so if your numbers can use up their set purchase quantity there's no waste and you get a better price. If you want them personalised there'll be a one off charge for a tool to be made for your logo etc.