There a quite a few things to consider when deciding whether to vaccinate or not, in the first instance, the number of birds you intend to keep? A flock of 50+ or just a dozen? Obviously more than 50 poses more husbandry and rearing issues compared with rearing a small flock of maybe a dozen layers. Particulary important to think with large numbers is the rearing process itself, young batches should be reared on together and not mixed for example.
If you choose to go down the route of vaccination it is advisable to get your hatching eggs from clean and healthy stock as vaccination schedules start from day old chicks.
Think about your set up.. Closed flock meaning no birds in..versus buying in stock from various suppliers/auctions and shows. Assess potential exposure to disease, meaning risk of infection to your birds, obviously small numbers of birds kept in a closed flock brings down the potential of infection considerably thereby you would in those circumstances question the need for vaccinating your birds.
The value of the birds is an important factor, by that I don't mean just in monetary terms. If the breed is hard to source, or you have been breeding one line for a number of years, what would happen if an infection decimated your flock? Could you easily restock? Some diseases like Mareks even when depopulating are very hard to remove. These are all things to think about.
Then if you decide to vaccinate, what to vaccinate against is the next question. There are a few common ones which are recommended in the UK specifically:
Mareks
Infectious Bronchitis
Mycoplasma
Coccidiosis
With regard to cost and doses, most vaccines are reasonable, Mareks is only around £9.00, and the most expensive is Mycoplasma which is about £40. This does not include the cost of prescription. They all come in 1,000 doses but I wouldn't let that worry you, just means you have a lot left over! You can share costs by timing your incubation with another breeder, as long as they are nearby as most vaccines when reconstituted have a short lifespan around 1 -2 hours.
It would also be a good idea to read up on how to administer the vaccine, there are a few courses around I believe and the link below is a very good guide. Most vaccines failures or "breaks" can be tracked down to poor vaccination technique. It is worth pointing out that vaccines are not 100% effective, however they go a long way to reducing the spread of the virus by reducing its pathogenic nature.
http://www.chickenvet.co.uk/services/vaccination-advice/index.aspx
At the end of the day the decision is absolutely yours, I choose to vaccinate however not many breeders do actually I hardly know of any. Bear in mind that equally not many breeders will have the experience either.