Looking to save money on batteries is not recommended - your whole investment in time and money depends on them. The advice against soldering is valid, but there are other considerations.
First of all, for 2500 mAH batteries, you will need a charger capable of 250 mA for the normal overnight charge. You may be told that your 50 mA charger will do the job in 50 hours (2500divided by 50) but that is not true, a full charge will probably never be reached.
A question is do you really need 2500 mAH? The great majority seldom need over 1000 mAH for a days activities. See www.Cermark.net for a more reasonable 1400 mAH 6V pack with welded connections and the proper harness for under $20. Also smaller and larger fresh Sanyo packs. Gud luck.