On the hole, the general rule of thumb is the fewer keywords or less competitive industry will result in much more affordable prices. This mainly reflects local businesses. Larger corporations who want to target more keywords and want to rank both at home and abroad will be paying more money to reflect the extra workload that will be required.
The other factor that will affect the SEO budget will be the geographic area being targeted. If you are targeting a local village or town, for example, this will require much less investment than London or the whole of the UK or international rankings. If you would like more information we are very happy to discuss the options with you as well as the costs which, as outlined above, will depend on the keywords you will be targeting as well as geographical areas.