supposing that; if we assume — a hypothesis set up for argument's sake
もし宝くじが当たったとしたら、何をする？
moshi takarakuji ga atatta to shitara, nani o suru?
If you won the lottery, what would you do? casual
この計画が本当に実現するとしたら、予算はどのくらい必要ですか。
kono keikaku ga hontou ni jitsugen suru to shitara, yosan wa dono kurai hitsuyou desu ka.
Supposing this plan actually happened, about how much budget would we need? neutral
Variants
とすればtosurebaformal
ば swaps in for たら — stiffer and more written, at home in essays and reasoned argument; interchangeable with としたら in most contexts.
とするとtosurutocolloquial
shifts weight onto what follows as an automatic inference — "that being the case, then…" — reading even more like a step in a logical chain than としたら／とすれば.
としたら
としたらto shitaraN3neutral