I had a quick look over "Incumbent and Challenger Campaign Spending Effects in Proportional Electoral Systems: The Irish Elections of 2002" by Kenneth Benoit and Michael Marsh in the current issue of Political Research Quarterly. It seems kind of interesting and thought I'd flag it up. Here's the abstract:
Positive effects of campaign spending on electoral outcomes have been found in several comparative, multiparty contexts, but very few of these systems use proportional representation. The few studies examining spending effects in multiparty elections have found that incumbent spending is no less effective than challenger spending, contrary to the vast bulk of empirical literature drawn from single-member district contexts. This study reexamines incumbentâ€”challenger differences in spending effects under the single transferable vote. Examining the Irish general elections of 2002, the authors find a positive and statistically significant relationship between spending and votes. Candidates that spend more win more votes, and outspending oneâ€™s rivals means winning more of the vote share. Spending more also directly increases a candidateâ€™s chance of winning a seat. Finally, incumbent spending is considerably less effective than spending by challengers from other parties but no less effective than spending by challengers from a candidateâ€™s own party.