@shylatte It is expensive to be a Jew whether you are haredi or not. Your level of observance increases the costs.
Synagogue fees and burial fees- mine is £150 per year(young adult) but it will rise to £1k per year when i have a child in the religion school so DH and I spend £300 per year together, MIL's is a few hundred pounds a year as orthodox synagogues tend to be cheaper as I guess the orthodox denomination is bigger and synagogues get support from United Synagogue. Also I think richer members probably donate more.
Housing- Most Jews live in North London/Herts. I was debating between borehamwood and East Finchley when buying my property it woudl have been 400k for a 2 bed house in Borehamwood or a 2 bed flat in Finchley. I chose the flat in Finchley but even if i had moved to borehamwood which is meant to be the 'cheaper' option, it would not have been less expensive. Stamford Hill is fairly well connected and has similar prices even if it is more grotty. Basically for many Jewish areas, you are looking at 700k to a million for a house (MIL's house is worth 700k) and its quite important to live in a jewish area for the facilities.
Education: there are state jewish schools but you need to pay for the religious studies element. Its not a lot (a few hundred?) but if you have several children, I suppose it adds up. Theoretically you can not pay but i guess its not very nice. And of course haredi schools have fees
Kosher food (esp meat): the cheapest way to be jewish is to be vegetarian. You can buy veggies from aldi and check for bugs. Many Jews however love their meat and kosher meat is more expensive. Kosher restaurants are more expensive not simply because of there are fewer of them but also cos a Jew needs to supervise.
Weddings: Once read that no Jew elopes to Vegas and gets married with 2 people as witnesses. If you have more family members, it is more expensive to have a wedding, particularly if you need to cater for kosher food for all of them.
Clothing: Religious Jews dress more formally, men wear suits (esp the haredim who do so daily and even regular orthodox wear suits on shabbat), women wear modest clothes that often would not look out of place in an office. Such clothing is more expensive than a £3 sun dress from boohoo so they spend more on clothing overall. They also attend more weddings and bar mitzvahs, so have to be suitably attired for them.
Religious ritual items/misc: you need tefillin for the men, a mezuzah for every door in your house (scrolls can be quite expensive at £30 each plus you need a case), candle sticks, challah covers, kippah, kiddush cups,religious books (my MIL has a whole wall of books). More kitchen supplies. And on passover, you can only buy kosher for passover products and those seem to be more expensive than the usuals (or maybe i am just imagining it).
Charity: Jews have to give to charity even if they are very poor.