A stock i7 920 has full load temp of around 74-77C under stock Intel cooler. But your stock 920 with only 5% load has 70C? Something is not right for your system.
If your system is a Dell XPS with an i7 920, then I am almost certain that the Dell heatsink is not a whimpy one. It should be much better than the stock Intel one. Dell does not use cheap CPU cooler for their XPS line with i7 920, AFAIK.
The only likely reason for such high temp (5% load is almost like idling) is:- the currently installed heatsink has bad contact and/or the thermal paste has dried up and/or the fins are totally clogged up with dust.
Solution:- Just take it out + reapply thermal paste + clean out all dust and then remount it properly. This should take it back to an idling temp of around 35C.
If you do want to get an aftermarket cooler, then I suggest Corsair A70 as your budget is US$50. A70 is currently selling at US$35 (after MIR) at Newegg.