I wouldn't say that it's modifying anything, it's an oblique argument or object.
An oblique argument is an argument of a relation that is marked with a preposition. (Syntactically, oblique arguments aren't direct arguments; that is, they aren't subjects or direct objects or second objects).
And, from SIL:
An oblique object is a grammatical relation proposed for a noun phrase clause constituent with the following characteristics:
- Its nature and behavior are more readily describable in semantic terms than syntactic.
- It is likely to be the most constrained in the semantic roles it may individually express.
- It is likely to be marked by an adposition or case affix.
To answer your other question: the most felicitous expansion, I would say, is
The Martins sold their house in order to get about 1.4 million pounds.
However, and I admit my limitation here: it doesn't quite sound right that way. Can't put my finger on it.