http://articulo.mercadolibre.com.ar/MLA-602979566-protec-osciloscopio-20-mhz-modelo-p-3502-c-_JM is a more or less typical cheap used analog oscilloscope: the 20MHz Protek P-3502C. The price is AR$3500 = US$232.
http://www.tek.com/oscilloscope/tbs1000b-digital-storage-oscilloscope Tektronix’s cheapest current scope is this TBS1000B line; the lowest-end scope in the line is the TBS1032B, with 500Msps and 30MHz analog bandwidth, for US$450, with a recording length of 2500 points.
http://articulo.mercadolibre.com.ar/MLA-620039875-osciloscopio-hantek-6022-be-20-mhz-_JM is a typical USB oscilloscope: the Hantek HT6022BE20MHz: 20MHz, AR$5000 (=US$331), 1MΩ 25pF input impedance, 48Msps, 8-bit resolution, 20mV to 5V gain range.
https://www.seeedstudio.com/item_detail.html?p_id=736 is the Seeed Studio DSO Quad 4-channel Digital Storage Oscilloscope, which costs US$169. It claims 72 Msps but doesn’t make any claims about analog input bandwidth on its two analog input channels. IIRC Seeed is an open-hardware shop; they do publish DSO Quad schematics and the Wiki lists user apps and it seems like they’re using an AD9288BSTZ-40 for their ADC.
It seems like you ought to be able to meet or exceed the capabilities of the Hantek unit for a much lower price, especially using recycled chips. I mean basically this is two high-speed ADCs (or one with a demultiplexed input) hooked up to a USB interface in a metal box, right?
Three candidate ADCs are the US$3.69 TI ADS830E/2K5, the US$4.50 Maxim MAX19505ETM+T, and the US$3.28 Analog Devices AD9283BRSZ-50, which last is available in quantity 1 from Digi-Key at US$6.01.
The AD9283 family is 90mW, has 475 MHz analog bandwidth, a 46.5 SNR, a 1V p-p analog input range, and runs off 3 volts. “Low-cost digital oscilloscopes” are explicitly called out as a use for the thing in its datasheet; Matthew Lai designed such a scope based on a slightly higher-speed member of the family. It claims ENOB of 7.5 bits at 27 MHz input, which is better than the Hantek unit.
Its input capacitance is only 2 pF. It says its input resistance is only 7 to 13 kΩ, which seem to be pullup and pulldown resistors to the power rails. So you probably need some kind of preamp, like maybe an opamp or something, to get high input impedance.
Lai’s design uses an FPGA to buffer the digitized signal. The Seeedstudio design uses both an FPGA and also an ARM Cortex microcontroller.
Lai’s design's analog front end was as follows:
Input goes into a 1MΩ metal film resistor in parallel with 20pF ceramic cap (not sure about this). Buffered by a Texas Instruments OPA656 wideband op amp with JFET input (very high impedance, which we need, because the probe has high input impedance. Downside? bloody expensive).
There are a variety of OPA656 parts; Digi-Key has the OPA656U at US$10.69, but I think he may have used a slightly more expensive one, but maybe the price has come down since 2010. The OPA656 family seems like it might be somewhat overkill for this, with a 230MHz bandwidth and gain-bandwidth product, 70 mA output current (!!), and 65dB open-loop voltage gain.
Lai’s total BOM cost is US$107.58; a quarter of this is these op-amps, and another quarter is the THS7002 preamp.