Hi Lucie
Statistics suggest that people tend to spend on average 5-10% of their budget on flowers, but that depends hugely on how important they are for you.
If you're getting married in a Church it's worth asking what/if theire flower guild can provide. Quite often something fairly neutral will be put together for you, and anyone else getting married the same weekend as you. If not, and you want to provide something, I would usually suggest a couple of pedestals that can be moved from the Church to the reception venue afterwards. It's nice to have something at the entrance to introduce your themes to your guests, and something by the altar. I wouldn't worry too much about having anything where you will be signing the register, it works just as well to put your bouquet down on the table. Pew ends are lovely, as long as the aisle is wide enough to accommodate 2 pew ends, you and whoever is giving you away. Some aisles are really narrow! Also, there is no need to have a pew end on every pew; alternate rows looks just as effective. You can even buy fanastic holders called 'clip-its' which have a pew end hanger that snaps off, enabling you to sit the arrangement flat and use them as table decorations after the service if you want to.
In terms of venue decoration, the cost of table centres etc varies hugely, according to your location, florist, the time of year you're getting married and the size and content of your arrangements. Realistically, your florist should be able to design a minimal table centre for under £20 rising to over £100 - it really all depends on the look you want to achieve and whether you love flowers enough to want to justiy the expense.
Not sure if that raises more questions than it answers, but hope it's a start!!
Georgie