I'm not convinced that either genitive of value or ablative of price is particularly appropriate in this context, in spite of the fact that the original English would seem to invite one or the other of them. (I could be wrong though!) Therefore, I'm going to suggest a few different approaches.
Although I've used some flowery vocabulary in these, so that they reflect the passages that were their inspiration, all of them could obviously be stripped down somewhat and made more prosaic (for example, quaestus instead of compendium; a simple form of minor instead of adjectives such as leviore or minus ubere).
triginta milia librarum? multi in aliorum exitium* multo leviore compendio ducti sunt!
Thirty thousand pounds? Many have been led to the murder of others by much less substantial profit!
* Or caedem or necem.
This is inspired by Seneca the Younger, Dialogi 4.8.2:
inter istos quos togatos vides nulla pax est: alter in alterius exitium levi compendio ducitur; nulli nisi ex alterius iniuria quaestus est; felicem oderunt, infelicem contemnunt; maiorem gravantur, minori graves sunt; diversis stimulantur cupiditatibus; omnia perdita ob levem voluptatem praedamque cupiunt.
Here's another:
triginta milia librarum? longe minoris pecuniae spes multos induxit ad necandum!
Thirty thousand pounds? Hope of far less money has led many to murder(ing)!
This is inspired by pseudo-Cicero, Rhetorica ad Herennium 2.29:
omnes enim cum minima peccata cum causa suscipiunt, tum vero illa, quae multo maxima sunt maleficia, aliquo certo emolumento inducti suscipere conantur. si multos induxit in peccatum pecuniae spes, si conplures scelere se contaminarunt imperii cupiditate, si multi leve conpendium fraude maxima commutarunt, cui mirum videbitur, istum a maleficio propter acerrimam formidinem non temperasse?
A couple more, for good measure...
Since the £30,000 (or a lot less) is the motivating factor – that is, the reason for or cause of the murder – another possibility is to use causa:
triginta milia librarum? quanto* minore causa** neces illatae sunt!
triginta milia librarum? quanto* minore causa** homines necati sunt!
Thirty thousand pounds? Murders have been committed/People have been murdered for much less cause!
* Or multo or longe.
** Or de causa or e causa.
Cf., for example, Pseudo-Quintilian, Declamationes maiores 6.2:
quanto minore causa excaecata est!
Finally, since obtaining the £30,000 (or a lot less) is the murderer's goal (so that 'for' doesn't so much express a notion of price or value as it is shorthand for 'for the purpose of obtaining'), a gerund/gerundive showing purpose can be used:
triginta milia librarum? ad compendium longe minus ubere nanciscendum neces illatae sunt!
triginta milia librarum? ad compendium longe minus ubere nanciscendum homines necati sunt!
Thirty thousand pounds? Murders have been committed/People have been murdered in order to obtain far less abundant profit!
This is inspired by Suetonius, Life of Vitellius 2.1:
contra plures auctorem generis libertinum prodiderunt, Cassius Severus nec minus alii eundem et sutorem veteramentarium, cuius filius sectionibus et cognituris uberius compendium nanctus, ex muliere vulgari, Antiochi cuiusdam furnariam exercentis filia, equitem R. genuerit.