A big spender is someone who:

• Frequently makes large purchases, especially of extravagant goods. ^{wiktionary.org 1}
• Spends money freely and in great amounts, especially on nonessential things or events. ^{thefreedictionary.com 2}
• Can also be used sarcastically to refer to someone who is reluctant to spend money, or who offers to spend only a small amount. ^{thefreedictionary.com 2}