I have a Poulan 2150 chain saw which needs a new chain oil pump. What is the cost to replace it. Should I just buy a new saw.

A new chain pump kit definitely cheaper than a new saw $15.00. Below is a link with the kit for the saw Poulan 2150. Provided to you as reference I don't garrantee site or lowest price. If this has been helpful please rate answer. Good Luck and have a great day. Jeff

Part #530071259

If you perform the labor repair likely <$20 (<$9 for pump, <$1 worm gear). Compare replacement cost to 34cc± replacement, your call.
A link to a Poulan 2150 Gas Saw Type 7 follows (may not be your saw):
http://www.ordertree.com/modelinfo/PWE/2150-Gas-Saw-Type-7/75.2150-Gas-Saw-Type-7.1.59.1.html

If the link is not your saw it can be found here.
The IPL (exploded parts list) for your saw is the only detail for the oil system and it is sketchy at best. Oil components and part numbers are listed but may be on different illustrations. They are also beneficial with disassembly/reassembly:
Poulan.com
http://www.ordertree.com/modelinfo/POULAN-WEED-EATER/75.59.html
The dash number of your serial number is the type.
The oil pump is under the outboard clutch, this is helpful with removal.
R² Drive Link Outboard Clutch by Drivelink.blogspot.com
http://www.youtube.com/watch?v=cdqkIuVNUQc

1. Check the IPL for your saw to see if any special tools are needed. Poulan (Craftsman, Jonsered, Husky) clutch removal tool part number 530031112 if needed, is under $5 from many sources (look at the visible area between the weights if 2 small round sockets are visible you need the tool).
2. There are metal piston stops available, when square (almost never) with the piston top these work well, if not square they have been known to punch through the piston. I suggest a length of nylon rope (nothing left behind) be used instead of the piston stop (retain 6" ± so you can remove it). Make sure the piston is near the top of the cylinder before feeding the cord or it can fall through the exhaust port and damage the piston as it rises.
3. Everyone I have ever removed had a left handed thread (tighten it to loosen it).