I can't help much with a specific cost per week as I'm sure the cost of things are different here in Australia however I can help break it down.
The day to day costs would be bedding and food. Research the kind of bedding you want, disposable bedding has a higher cost in the long term but is cheaper at the get go, reusable fleece bedding has a high starting cost but the ongoing cost is only the cost of washing it.
For food they would need 1 cup of veg per guinea pig per day, 1/8 of a cup of pellets per guinea pig per day, and unlimited grass hay.
Vegetables are what you will be buying the most frequently, good everyday vegetables for one guinea pig are 1/8 of a large bell pepper, 1-2 leaves of red/green leaf lettuce, a 1 inch slice of a zucchini, a 1 inch slice of a carrot, 1-2 cherry tomatoes. There are also more vegetables that you can feed, but they can't be fed on an everyday basis (the link to a vegetable chart can be found in one of the links I will link at the end of this post). So if you have a pair double that and you should be able to estimate how many vegetables you will need.
A 5 pound bag of pellets (from Oxbow or KMS, the only brands we recommend) should last a little over 2 months for a pair of guinea pigs.
Hay is where costs can vary greatly depending on where/how you buy it. The cheapest way by far to get hay is in a bail from a farmer or fodder store, however you would need to be able to store the hay and would need to check the quality of the hay before you buy it. The second cheapest way is to buy bulk boxes online, (from places like KMS, Small pet select, sweet meadow farms, american pet diner) the larger amount you buy the cheaper it will be per pound. The most expensive way by far to buy hay is in those little bags at a petstore or places like walmart. Given hay is 70% of a guinea pigs diet and they do tend to waste quite a bit no matter what you do the more money you can save while still getting quality hay the better.
However the biggest cost you should be planning for is any vet care they may need. Having a good amount of funds set aside for vet care expenses is always a wise decision. It can help greatly to find a good exotic vet (regular small animal vets aren't trained to care for guinea pigs) who sees guinea pigs (ideally regularly sees guinea pigs) ahead of time and enquire as to the cost for a consultation (and perhaps even the estimated cost of something like an x-ray). The costs can vary greatly from one clinic to the next even in the same city.
Another piece of advice I can give is to consider adopting guinea pigs from a rescue or shelter. Not only would you be freeing up space for them to take in more unwanted guinea pigs but guinea pigs from a good shelter or rescue are less likely to already be sick or pregnant than those purchased from a pet store.
As it sounds like you haven't owned guinea pigs for a little while it may be good to have a read of these:
https://www.guineapigcages.com/forum...nea-pig-owners
https://www.guineapigcages.com/forum...ur-guinea-pigs!
