Cost the whole project: if you are going to go the DIY route (which is certainly possible, especially if you are experienced in bigger home improvement projects, and if your loft is simple and easy to board) you need to budget for all the materials, of which the most important are the ladder, the boards and (if it is part of your project) insulation. If you are going to use professional loft specialists, get a quote with detailed specification of the work to be done and materials used.
When getting a quote, check if the firm you are considering offers a guarantee for their work, and how long, so you can be sure of peace of mind for years down the line.
And finally… even if you are almost-sure that DIY is the way to go, do get a quote — or a few. You might be pleasantly surprised at how affordable such a service is. And if you take into account the time needed to do the work (we all know that old but true saying that time is money, sometimes literally) and all the associated hassle of getting the materials, doing the work and tidying up, hiring professionals might turn out a much better deal than the DIY option.
Reputable loft specialists will provide a free quote, and often offer deals for storage conversion packages, which means that the bigger the project, the more advantageous is to hire professionals.