Having a new website created or having amendments made to your existing site can be costly. What may seem like a small job to you could require hours of programming and therefore come with a large price tag. It is important to understand what costs are involved and what elements are the most costly. You need to ensure that you have a budget in mind and that know if you have any flexibility on that. If for example you have a maximum of a thousand pounds then you should make that clear when agreeing to any additional work.
Often additional costs spring up when the brief has not been fully understood. It is important to give the design and programming team as much information as possible as even send them examples of sites that you like.
If you feel like the costs are spiralling then arrange a meeting with your web agency to discuss what is happening and how you can keep costs low. They may tell you that you need to drop a certain element of the site if it is not going to fit within your budget. If this is the case, then it can always be added back in at a later date once you have more money to invest.