The Ulster Unionists spent more on the May assembly elections than other Northern Ireland parties, according to Electoral Commission figures.
The party spent more than £96,000 on its campaign and secured just 16 seats.
The DUP spent more than £84,000 and won 38 seats.
The next biggest spenders were Sinn Fein whose campaign cost nearly £52,000 - they got 29 seats. The SDLP spent more than £38,000 and ended up with only 14 seats.
Alliance spent nearly £29,000 and secured eight seats.
The 13 parties which submitted their returns on time spent a total of £338,739 during the regulated period from 6 January to polling day on 5 May.
Just over £100,000 of this went on party political broadcasts and a further £96,000 on advertising.
Among other parties, the British National Party spent £3,181, Traditional Unionist Voice £6,413 and the UK Independence Party (UKIP) £18,176.
The Progressive Unionists, who failed to win a seat, spent just £310.
One party, The People Before Profit Alliance, have not yet submitted their return to the commission.