Hiya,
I'm not yet pregnant, so I can't offer a lot of advice on this site, but Guides I do know about! Currently I run a Guide unit (ages 10-14) but I used to run a Rainbow unit and I've helped with Brownies a bit.
The exact cost will vary depending on the major cost to any unit, which is normally rent, and the number of girls you have to share the costs between. Units that meet in a Guide-owned building or a church or village hall might be lucky enough to have cheap, or even free rent, but units that meet in school halls often have to pay more because the rents are fixed by the council and you have to pay towards having the caretaker unlock etc. A pound per meeting seems to be average among the units I've worked in. Many units will ask for the money to be paid termly or half termly, but others ask you to pay each week. Paying termly allows units to budget more easily, especially because we have to buy enough craft materials for example, for every girl to take part in every activity, regardless of whether the girl actually turns up at the meeting. Having said that, there should never be a case whereby a girl is not allowed to take part in an activity or to join at all, if money is the issue. If a parent spoke to a Guider and explained that paying termly, for example, would be difficult, then some other arrangement can always be made.
You will probably find that you are asked to support some fundraising efforts during the year as well. All units have to pay subs to the Guide Association on behalf of every girl and adult working with that unit - this year it was just over 15 pounds per person. That is a very large bill when you are only charging one pound per girl for 30-35 meetings per year. Some units run fundraising events, others, especially in areas where the average parent is "cash rich, time poor", just ask parents to pay a direct contribution, either as a one off or spread over the three terms. Some units just add it into the per-week charge. The cost is about the same for all age groups - the older the girls, the more expensive the activities can be, but there are usually more girls to spread the cost between.
Rainbows normally meet for about an hour, Brownies for an hour and a half. Again, this varies from unit to unit. Rainbows often meet early, soon after school - some units even meet on Saturdays. It really depends on when a meeting place is available and when the adults are available to attend. If the leaders work, the meeting usually can't be until later.
Kbear was right about the activities - crafts, games, songs, other activities depending on the age group. Brownies go away on residential holidays, if the leaders have passed the right qualification and can spare the time. If you are interested - put your daughters' name down quick. Schools are usually a good place to get the details of the meeting times or the leaders phone number - most of us give our numbers out freely, or if you give your details to a parent of another Rainbow or Brownie to pass on, we'll call you. Some units have long waiting lists, others have some vacancies most of the time - so much varies between units that I can't be more specific.
I've gone on long enough, but if you want to know any more, let me know.
Happy to help!