I also love your mom's response!!! I might have a similar response...something of the tune of "HA! I love your sense of humor! I'd like a years worth of oil changes and gas for our family car:) But seriously, what do you want that we could actually afford?!"
Part of me thinks they would feel bad if they knew how their list came across so if you are honest and laugh at it, it will not only let them know without it being a heavy topic of shame, but it might make you feel good too. Because, really, it is sort of funny when this time of year comes rolling around and people get so caught up in the gifting that they forget about budgets for self and others:)
Also, you sound like such a fun x-mas gift person! I'm impressed by how on top of it you are...I'm that loser desperately looking for gifts 2 days before x-mas and who just grabs anything I can afford and seems right for the person. Then, and this is awful I know, I try to think up a story of "why it's so perfect" for the person:) Yeah...I'm that loser!