If you're so strapped for cash, wait until the Steam sales.
Just Cause 2 is always 75% off (£3.49) in these events, and I would wager that FTL will be discounted too (probably 50% off through the entire event, and 75% off for one day).
Though JC2 has been as cheap as £2.49 (Steam key from Gamersgate).
If you do buy FTL before the sales, buy it direct from the developer's website in US$ using paypal; it'll cost a little less ($10 = £6.44) and you get both a DRM-free copy AND a Steam key. (plus the developer gets a larger share of the revenue)