I got bored of slipping around on an Alpkit mat and eventually getting a numb bum... I also couldn't afford to buy a decent underquilt, neither do I have the expertise to make one.
What has worked wonders though, is an Exped UL down sleeping mat that I usually use for bivi camping (in small size) inside my Vango Viper 750 sleeping bag. It fits perfectly, and leaves just enough room at the end for a pillow should you want one. (I use an exped air pillow as it's amazingly comfy!)
Because it fits so well, the sleeping bag stays in one place when you turn in your sleep. (which I do often) I can definitely recommend seeing how well your mat will fit inside your sleeping bag for anyone that can't afford a decent underquilt!
Not to mention, my Exped down mat is less than 1/3 the size of my friends underquilt when packed. (although he hasn't bought a compression bag yet, so I might be retracting that statement very soon...)