I felt very extravagant是系表结构；spending £100 in a dress是现在分词短语作原因状语或时间状语。

虽然extravagant可以作表语，表示一个人花钱或使用某物过度。例如词典例句：

On my income, I can't afford to be extravagant. [=I can't afford to buy expensive things that I don't need]（韦氏学生词典）

Don’t be too extravagant with the wine.（朗文）

Someone has been very extravagant with paper. （麦克米伦）

Residents were warned not to be extravagant with water, in view of the low rainfall this year. （牛津）

但我觉得《牛津词典》的如下例句中，extravagant不是表语，而是宾补。

I felt very extravagant spending £100 on a dress.

feel为复合宾语及物动词，意为think，believe。动名词作宾语较长，后移至宾补之后。动名词后移不同于不定式或that从句，通常不需要形式宾语it。

该句等于：

I felt that spending £100 on a dress is very extravagant.

即本句中形容词是描述动名词的，而不是描述主语的。

我觉得花100英镑买一条连衣裙太奢侈了。

I(主语) felt（系动词） very extravagant（表语）（in省略) spending £100 in a dress（方面状语）.

参考译文：我感觉在一件衣服上花费£100是非常奢侈的。