The cheapest but easy and effective way to get a plasmid miniprep kit is to buy spin columns for plasmid miniprep alone and make solutions with the following buffer recipe:

Buffer P1: 50 mM Tris-HCl, 10 mM EDTA, pH 8.0 (25°C, the same below), 50 ug/ml RNase A
Buffer P2: 0.2 M NaOH, 1% SDS
Buffer N3: 4 M guanidine HCl, 0.5 M potassium acetate, pH 4.2
Buffer PB (washing): 5 M guanidine HCl, 20 mM Tris-HCl, pH 6.6, 38% ethanol
Buffer PE (washing): 100 mM NaCl, 10 mM Tris-HCl, pH 7.5, add 4 volume ethanol before use
Buffer EB (elution): 10 mM Tris-HCl, pH 8.5

The protocols of various commercial plasmid miniprep kits are similar.