Tri-tip is cut from the Bottom Sirloin. Can't say for sure where your Sirloin Tip Roast is from.
If it is the same thing, that is an exceptional price on it. They go for almost twice that around here (when I can find them).
New Braunfels Hondo Offset
New Braunfels Bandera (Refurbished, Modified & Improved)
32 year old Char-Broil 450
1996 Blue Weber SS Performer
2003 Black Weber SS Performer
Weber 22.5 OTS converted to OTG with rotisserie & Smokenator
Weber SJS & modified stock pot Mini-WSM
STOK Island gasser