Excluding decorations, I've picked out the most affordable products for both a DIY set up and a kit, and compared them both. As you will see below, the larger the tank, the more cost effective per gallon; with 10 gallon tanks being extremely cost effective vs 5 gallon tanks. Also, the larger the kit, the less cost effective it is vs setting up your own tank.

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5 Gallon Tank

	DIY	Kit
5 Gallon Tank	$10-$15 @ Petco	$31.99
Filter	$1.99 + $6.99 = $8.98	included
Heater	$11.29	$11.29
Thermometer	$2.29	$2.29
Substrate	$5.99	$5.99
Decor	-	-
Water Conditioner	$6.49	$6.49
Top	$9.44	included
Light	$15.99	included
Net	$1.93	$1.93
Total	$72.40	$59.98

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10 Gallon Tank

	DIY	Kit
10 Gallon Tank	$10-$15 @ Petco	$44.99
Filter	$6.99 + $6.99 = $13.98	included
Heater	$11.29	included
Thermometer	$2.29	$2.29 (do not use strip thermometers)
Substrate	$12.99	$12.99
Decor	-	-
Water Conditioner	$6.49	$6.49
Top	$12.57	included
Light	$15.99	included
Net	$1.93	included
Total	$87.53	$66.76

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20 Gallon Tank

	DIY	Kit
20 Gallon Tank	$20 @ Petco	$96.99
Filter	$6.99 + $6.99 = $13.98	included
Heater	$11.92	$11.92
Thermometer	$2.29	$2.29 (do not use strip thermometers)
Substrate	$12.99 x2 = $25.98	$12.99x2 = $25.98
Decor	-	-
Water Conditioner	$6.49	$6.49 (tiny bottle included)
Top	$15.73	included
Light	$15.99	included
Net	$1.93	Included
Total	$114.31	$143.67