From Webster's Revised Unabridged Dictionary (1913)

Purchase \Pur"chase\ v. t. [OE. purchasen, porchacen, OF. porchacier, purchacier, to pursue, to seek eagerly, F. pourchasser; OF. pour, por, pur, for (L. pro) + chacier to pursue, to chase. ]
1. To pursue and obtain; to acquire by seeking; to gain, obtain, or acquire. --Chaucer.

[Thus: "grant us grace... to gain your heavenly place" - AJ]