Ooh glad some one loves their Brown Owl!!!
You don't have to buy the trousers and all that. Usually the minimum is the t-shirt or long sleeved top, and perhaps the gilet or sash to put badges on but this is optional. We do expect at least the t-sirt so they are in uniform and are identifiable as Brownies.
As other have said subs vary greatly and some charge seperatly for the yearly 'census' others include this in subs, some charge a joining fee to cover the books and boxes they get as we can get girls joining getting all the stuff then leaving a month later when they decide it is not for them.
Holidays can cost from £25 for a local one, further away req buses etc will cost more. Out trip this year is costing £5.50, as we are doing our own transport.
The guiding association do state that they will not prevent a girl from taking part due to finacial contraints, so if money is a problem do talk to the guider in charge, often they will keep secong hand uniform etc.
The programme of activities can also vary a lot from pack to pack, depends on the skills of the leaders, some do loads of gmaes, or crafts, others do interest badges as a pack. I like to have a very varied programme and one week we might be doing a jelly babies them night the next working on our brownie traditions badge.
They should ba happy for your DD to go along for a couple of weeks to try it out and see if it is for her, don't buy any uniform straight away, until you are sure.