Yes it seems you want to use regression. Clustering is a good algorithm for understanding your data, but if you want to predict a value like Purchase Amount based on other variables then regression is the way to go. You said you wanted to 'point at a specific column (Purchase Amount) and ask DM to tell me what groups of fields lead to different purchase amounts ranges'. I didn't completely understand it at first because I might have stated it differently - 'I have several independent variables that I know about customers (such as gender, income, past purchase history) and I want to predict a dependent variable called purchase amount'. Based on this I would recommend you start with a decision tree as your algorithm.

To do this in Excel, load the data into a worksheet with column headers. Go to the data mining menu item (assuming you've installed the data mining add-in) and select the dataset. Click the Classify icon. It will walk you through the process, just make sure you select Purchase Amount as the 'Column to Analyze'. Continue through the wizard, it will generate the mining model and show you the decision tree. You can take a visual look at the tree and you should get a good idea as to its structure.

After the model is built you have to test it to see if it is a good predictor of purchase amount. Use the icons under the Accuracy and Validation section for this. Then go through the cycle again and again until you get good results.

By the way, funny you said doing this with Excel seems too simple. Actually this a very powerful way to do this work, since Excel is invoking Analysis Services in the background to build and test the mining models. You get the power of Excel and Analysis Services together in one package.