vegan food is actually cheaper than meat.
i know this coz i was extremely poor when i was a student, and my diet consisted mainly of vegetables, apples, rice, bread and lentils. nothing fancy like vegeburgers or quorn mince or soy milk. i would eat occasional poor quality minced meat and consume some milk and a little cheese. in every country there is a type of bean or lentil that is native to that country and is very cheap. in many countries cornmeal is cheaper than wheat, and there are always cheap vegetables in season (cheaper than meat anyway). in the uk cabbage, carrots and potatoes are the cheapest vegetables. i used to live on these.
people used to think i was vegan (i was a meat eater when i could get it!) and i wasn't even orthodox. maybe this is why i love fasting food now, coz i got used to it. even when i ate meat, i would add a load of lentils to make the meat stretch further.
this is why many poor people throughout the world eat an almost lentern diet throughout the year.
as for not loosing weight, it's hard not to loose a little, so try to gain 2 extra kilos before the fast, and eat plenty of chips (on days when oil is allowed) and plenty of nuts at other times.
i was eating some hazelnuts recently that were 67% fat! they were not cheap though, so maybe stick to the chips if u have a tight budget.
if you want to try fasting, then maybe try the coptic way - we get to eat oil every day so u can have chips all the time! we don't eat shell fish during lent though, it's totally vegan.
fasting is only advised as part of a 'diet' of prayer, Bible study, caring for the poor and church attendance, so i suggest u speak to a priest or orthodox friend (if u don't have a priest) for advice before starting.
when i started fasting, i started gently, by just cutting out meat but eating fish, eggs and dairy products - so it's good to take it slowly until u get used to it, as others here have wisely suggested.
may God bless u and guide u